The achievements in the game are overral farly easy, except maybe one or two that require a little more effort. Still I found them very manageable.
The guide is divided in three main parts: the achievements relative to the minigames, the achievements obtainable during the whole game, and the achievements relative to each specific level.
Have fun!

Achievement related to minigames can be obtained both during Story mode and in the Party mode. I'll list them in this section for utility and list them in their respective Story mode level in the other sections.

Who Finds a Friend Finds a Treasure
Play in party game mode.


Just launch the party mode from the menu.

Don't worry, I'm an engineer!
Take more than 30 seconds to fix a leak.


"Bananaboat" minigame, however I suggest attempting this one in Story mode on Easy difficulty, at the end of "The Village", the first level of the game. The reason for this is that on harder difficulties not reparing the leak on the boat in time will almost certainly result in a Game Over. Move Bud and Terence on the opposite side of the leak and wait patiently for thirty seconds, then repair the leak.

Don't touch my bananas!
Cross the river without losing any bananas.


Attempt this one in story mode on Easy difficulty! Otherwise, you'll be plagued by crocodiles, leaks and herons. There are four situations you can find youself into: stable situation, crocodile rampage, leak occurrence and heron roosting. Each situation must be treated differently.

Stable situation: since Bud is heavier than Terence, to properly balance the boat in this situation, they should be distanced accordingly, so if Terence is on one end of the boat, Bud should be some centimeters from the other end, but not exactly simmetrical to Terence (right picture, the one on the left shows a situation in which Bud is not properly positioned). This is the situation you should be aiming of returing as soon as possible after resolving one of the other three.


Crocodile rampage: both Bud and Terence should be on the other end of the boat, with Bud a little behind Terence; as soon as the crocodile leaves, Bud should hurry to the other end (still remaining some centimeters behind),

Leak occurrence: quickly move Terence to repair the boat, while keeping Bud some centimeters from the other end; as the leak is repaired, the boat will slowly sink: just move Bud slighty to the end of the boat for every centimeter the boat sinks until it gets repaired, then slowly pull back.

Heron roosting: use Terence to push the heron away since Bud's weight, combined with the heron's, will unavoidably sink the boat.

Won't you finish this one?
Steal food from the same opponent 3 times.


"Binge" minigame: by inputting a series of displayed commands, you can eat one of the four dishes shown. More than one player can attempt to eat the same dish (each of the directional inputs will be outilined by that player's colour), so let one player start the combo, than quickly finish it yourself to steal his dish. Very simple and will probably happen randomly.

Superfuzz!
Complete the training with at least 5 seconds left.


"Training" minigame, I obtained this one on my first try in story mode, on Hard difficulty without trying. Simply input the commands shown and don't shoot any hostages to avoid losing precious time.

Sorry, Boss!
Hit 5 hostages during training.


"Training" minigame: just press the button shown on the lady in pink during the shooting sequence five times.

Did you get it?
Win all rounds of the three-card game by always choosing first.


"Three Cards" minigame, the trick is to always follow your card by paying attention to the dealer's hands (the ring on his right hand is not there just for show). Be extra careful when he picks two cards into the same hand to remember if your card is the first or the second one.

Serrano Is Back
Win the pelota match without letting the opponents score.


"Pelota" minigame: incredibly, the AI can pull this one by itself, with a bit of luck. If you position yourself on the forward white line (the one nearest to the wall where the ball is launched) the AI will catch the ball 9 times out of 10. Simply try to score one point, and then, with a bit of luck, you can let the AI carry the match for you.

I think you cheated. Oh, you really think?
Cheat to take away a point from the opponents.


"Cards" minigame, just cheat everytime there are two cards of your colour (and only then) and you'll obtain this one without trying.

Internationally Acclaimed Singer
Complete the chorus without missing any notes.


"Choir" minigame: this one will be painful. You have to press the same button twice and then input a directional movement to move yourself among the chorus and escape from the otherside. Seems easy enough, right? Well it isn't. First of all you have to press that button (and that button only, otherwise it will count as a missed note) right when the outer circle ecompasses the button's icon, otherwise it will count as a missed note (luckily the button will flash to show you when you need to press it), then you have to remember to press the direction you want to move into, because if you remain still, you guess it, will count as a missed note. There are also three kind of chorus members who will obstruct you (outlined in red), and those cannot be swapped places with, requiring you to circumvent them: a trio of men who will stand still, a fat woman who will move from left to right and viceversa, and the most annoying one: a slim woman who will move up and down. Those women will let you know when they are going to move by starting to pull the member they will trade place with before moving. Another thing to pay attention to is that everytime Bud goes "Bah bah bah bah" (you get it if you watched the movie, otherwise, everytime Bud does the movement with his fingers on his lips) you'll get to move twice in a row, so watch out and don't input the wrong command.

In the end, just try to not lose focus and you should obtain this one after several tries.

That was some fancy shooting
Win the airboat challenge without getting hit.


"Hoverboats" minigame, but attempt this one in Story Mode, on Easy difficulty. Always be on the move and don't move in a straight line for more than two seconds and you should obtain this one in a couple of tries. The good thing on Easy difficulty is that your enemies are more likely to hit each other while attempting to hit you.


I would have preferred red with a yellow tarp
Win the airboat challenge by only driving backwards.


"Hoverboats" minigame, attempt this one in Story mode, on Easy. The main effort will be learning to move by driving backwars without getting stuck in the corners (otherwise the AI will ram you at full speed, trapping you and making escaping, or even moving, impossible and softlocking the game), otherwise this one is quite easy: stop when you need to launch a barrell and target the nearest enemy; most of the time, the AIs will knock each other out while attempting to catch you.

These achievements are not related to a specific level and can be obtained anytime during Story mode.

Turn the other cheek
Activate a ping pong combo with an enemy.


This one is easy enough and you'll probably obtain it without even trying: when the AI-controller player starts to attack an enemy with fast attacks, just position yourself behind said enemy and start hitting with fast attacks, so that both Bud and Terence are targetting the same enemy, one from the front and the other from the back.

Watch out the springboard
Hit an enemy using the lever on the trampoline.


The earliest you can get this one is in "The Pier" level, when you get introduced to item interaction: you'll notice a trampoline amongst the objects you can interact with; just push the trampoline with Bud while waiting for an enemy to wander just right under it to obtain the achievement.

Goal!
Hit an enemy by hitting an item on a counter with Terence.


This too can be obtained in "The Pier", in the exact same section as the the trampoline, you will notice on the left side and right side of the harbour some metal boxes with items on them: use Terence to jump on these boxes and start kicking these items until one enemy is hit by them.

Hold this for me!
Eliminate an enemy by throwing a heavy object with both hands with Bud.


There is a huge variety of heavy objects: heavy vases, rubbish bins, etc. just be sure to eliminate an enemy, not merely hitting them with one of these items to obtain the achievement.

Get down!
Eliminate a thrower opponent that can only be reached with Terence's aim shot.


For some reason, altough I eliminated multiple thrower enemies, the achievement only popped up in "The City" level, after eliminating one of the throwing enemies almost at the end of the level (the last normal fight before the boss battle).

These achievement can be obtained in the first level of the game "The Village".

Harder, guys!
Complete the throwing tutorial without taking any damage.


Not the easiest achievement: you'll be forced to use Bud for the first part of the fight, which is usually the hardest since his hitbox is larger than Terence's and he can't aim while launching objects. To make things simpler, do not try to attack an enemy while there are other enemies on the screen who still possess a throwable object: they will almost certainly hit you before you can hit one of them. Instead, grab an object, move continuously up and down to avoid getting hit, and as soon as all the enemies are disarmed, charge up your throw attack to hit the enemy right in front of you. After knocking out about five enemies, you'll swap to Terence: since Terence can aim his shoots by charging up his throw, and since these throws automatically aim the nearest enemy, the second part of the fight should be considerably easier and quicker.

Don't touch my bananas!
Cross the river without losing any bananas.


Don't worry, I'm an engineer!
Take more than 30 seconds to fix a leak.


Refer to the "Minigames" section.

There is only one achievement exclusive to the Pier, the second level of the game.

The box breaker
Avoid Wyke's crates.


At the end of the level you'll have a boss battle with Wyke and the Captain, Bud and Terence look-alikes. During the second phase of the battle, Wyke will start to manouver a crate to drop on your head: just hit him with Terence launch attack quick enough and he'll not have the time to even move the crate. Either way, it's fairly easy to avoid getting hit.

There are no achievements exclusively available on this level.

There is one achievement exclusivelly available in this level, and one achievement tied to a minigame.

The rocking chair
Eliminate an enemy with the rocking chair


You will find two rocking chairs (red and white striped): one just after the inital minigame and the other one immediately after the first fight; if you punch them, they will knock any enemy standing near them off. However, they do absymal damage, so you'll want to weaken one enemy first before attempting this. Note that even at pratically zero health, you'll require three or four hits to actually eliminate the enemy and obtain the achievement.

Won't you finish this one?
Steal food from the same opponent 3 times.


Refer to the minigames section.

There are two achievements that can be obtained in this level, both are tied to a minigame.

Superfuzz!
Complete the training with at least 5 seconds left.


Sorry, Boss!
Hit 5 hostages during training.


Refer to the minigame section.

There is one achievement exclusivelly available in this level.

Who you gonna call? Not you!
Hit an enemy on the pole.


Right at the start of the big fight, on the right side of the gym, you'll notice a huge pole, used by enemies to lower themselves on the floor level. Just hit one of these enemies before they manage toreach the floor and you'll obtain the achievement.

There are three achievements related to this level.

Chinese lanterns
Eliminate a Chinatown enemy by using the lanterns with Terence.


Scattered in the level, there are green street lamps: Terence can interact with them to execute a powerful kick that will heavily damage anyone unlucky enough to be in range. Eliminate an enemy with this ability and you'll get the achievement (you'll get this even if AI-controlled Terence manages to defeat an enemy this way).

The gardener
Talk to Alex the gardener.


At the end of the level, you'll have to reach the rooftop of a building to change your clothes. You'll notice a Bud look-alike who's cutting an hedge. Talk to him and you'll get the achievement.

Did you get it?
Win all rounds of the three-card game by always choosing first.


Refer to the minigames section.

No achievements are exclusivelly available in this level.

There is only one achievement exclusivelly available in this level, and it's relative to a minigame.

Serrano Is Back
Win the pelota match without letting the opponents score.


Refer to the minigames section.

There is one achievement exclusivelly available in this level.

One at a time
Use Bud to throw an enemy into a turnstile.


At the start of the level, during the first big fight, you'll notice a series of turnstiles on the right side of the screen: simply knock down one enemy, grab them while using Bud and launch them (while keeping the launch button pressed) into one of the turnstiles to get the achievement.

There are three achievements available in this level, one is related to a minigame.

Starboy
Talk to the space child.


This is easily missable: just at the start of the level, before entering the gambling house, at the topmost part of the room, just before the bowling alleys proper, you will find a small kid playing an arcade: interact with him to get the achievement.

Strike!
Make a bowling strike by throwing an opponent with Bud.


When returning to the bowling alley after the Cards minigame, be sure to knock off an enemy and grab them, then launch them onto the pins to get the achievement. You have a small window of time to obtain this one for after you defeat the first batch of enemies you'll have to leave the bowling alley proper behind.

I think you cheated. Oh, you really think?
Cheat to take away a point from the opponents.


Refer to the minigame section.

There is one achievement exclusivelly available in this level.

No Dana, only Zuul
Awaken the Zuul.


In "The City" level, just before the boss battle (after exiting the maze composed by a series of balconies and confusing doors), when you have to climb the long ladder to reach the rooftoop, you can notice two ominous statues on the sides of the rooftoop you are standing on (the statues being the Terror Dogs from the Ghostbusters movie): hit both of them until their eyes turn red and you'll obtain the achievement.

There are two achievements available in this level, one is tied to a minigame.

He had it in for you
Don't get hit by the sacks.


Just after the Choir minigame, you'll have to fight your way trough the backstage, while some sacks are constantly being lowered on your head; luckily, they will aways target the same area and can be easily recognized by their shadows. You can easily completely the whole fight by staying on the left side of the screen, where the sacks are not dropped, and after that, when you'll have to advance, just run on the lower side of the screen to avoid getting hit. You'll get the achievement as soon as you get on the lift on the right side of the screen.

Internationally Acclaimed Singer
Complete the chorus without missing any notes.


Refer to the minigames section.

There are four achievements available in this level, two of them are tied to a minigame.

Kept you waiting, huh?
Cross the military base without being detected.


At the start of the level, after escaping your cell, you have a stealth section composed of three sections: you have to avoid getting spotted by the cameras' and guards' luminous cones of vision. This is easier said than done since your hitbox is quite buggy and most of the times you'll think you are out of range while in reality you are not.
The first section is fairly easy: just hide behind the crates to avoid the camera's cone of vision, and kick the cart to unlock the laser barrier obstructing you way.
The second section in the hardest one: the first guard will almost always spot you, and you'll have to be careful not to hit any of the metal boxes scattered around the floor to avoid getting detected (also avoid the chains hanging from the roof). To avoid the first guard get behind the two impiled metal crates and wait for the guard to turn your back on you: this is the most difficult passage since to remain behind the crates you'll have to constantly move forward to avoid the effect of the conveyor belt, trying to hit the sweet spot behind getting too far from the crate in one of the two directions. As soon as the guard turns his back, quickly move against the conveyor belt (do not attempt to leave it or you'll surely hit one of the metal boxes) and hide behind the large set of crates. These are far enough from the conveyor belt to not require you to constantly move. After passing the first guard, the second one is easy: get on the conveyor belt while he's turned, hide behind the crates, wait for him to turn again, and reach the final section.
The third (and final) section is easier than the last one, but harder than the first: hit the switch near the conveyor bel to activate it, then use the moving boxes as a mobile cover; then, using the cart itself as a cover, reach it, and push it to unlock the barrier, then reach the exit while still using the same cart as a cover again.

Heavy Machine Gun!
Free the "special" prisoner.


In "The Military Base" level, just after freeing agent Tufano, don't advance just yet and reach the cell left to the one where Tufano was being kept in: you'll discover the good old Metal Slug prisoner bound by some ropes, talk to him to free him and receive an heartfelt THANK YOU!

That was some fancy shooting
Win the airboat challenge without getting hit.


I would have preferred red with a yellow tarp
Win the airboat challenge by only driving backwards.


Refer to the minigames section.

There is one achievement relative to the Cuba level and two achivements related to completing the game, I'll list them in the same section.

Monkey Kong
Avoid the rolling barrels.

At the start of the "Cuba" level, just after crossing the street you'll have to climb a slope while fighting enemies; there will be an endless barrage of barrels from the right side of the screen. To avoid them, just walk on the sidewalk on the upper side of the screen until you reach the top; there, you'll find three goons launching barrels from the top to the bottom of the screen. Select Terence since his hitbox is smaller, and simply run past them when you see an opening.

Are we done? No!
Complete the game.


...or maybe yes? But I Decide!
Complete the game in hard mode.


I found the game easier than the first one, even on hard difficulty, and I'm not very good at playing beat 'em ups. The only real difficulty I had was with some minigames (looking at you, Banana Joe) and the final fight, which I didn't properly understand at first, but became manageable after a few tries.

29/09/2023: First draft of the guide published. Just Minigame achievements covered, for now.
29/09/2023: Started working on the other achievements, covered nine of them.
30/09/2023: Properly completed the first draft of the guide. Added description for every achievement except Starboy.
03/10/2023: Rearranged the sections and added images for every achievement. The guide is complete except for the Starboy achievement.
Just some minutes after, I finally found the damn boy! The guide is now REALLY complete!