Ok, if you play Doom with default bindings, you're most likely to not do so good. Because anything further that 5 will require you to take your fingers of WASD.
This is optionally, but here are my bindings I use if you're interested on using these.
1 = Shotgun
2 = Assault Rifle
3 = Plasma Rifle
4 = Chaingun
5 = SSG
Q = Guass Cannon
F = Rocket Launcher
R = BFG
Use = Mouse3
Glory Kill = E

However, I would suggesting asking someone or use any browser to find your most favorite bindings.
But I mean, "you do you".

So, I switch between my mods like everytime i play, but my most used ones and even current are:
SG: Sticky Bombs, Charge Burst
AF: Micro Missiles
PF: Stun Bomb, Heat Blast
RL: Lock-On Burst
GC: Percision Bolt
CG: Mobile Turrent

Imps: Imps are one of the first enemys you meet on your journey. These little guys acts as a way to get ammo, health, or armor.
Possessed: Not much to be said about these guys, they acts like the less athletic Imps.
Hell Razers/Possessed Soldiers: These dudes and an "ok" level of difficulty. They are pretty annoying to me so I kill them before much others.
Hell Knights: These guys can be bullys. They are fast and use hard attacks on you, so stay at a good range then you'll be fine. Or use a Siege Mode shot on them.
Revenants: Not to much different then a Hell Knight but they could fly. But they can die in one shot of a Siege Mode shot so that's good. But stay far from them at all costs.
Pinkies/Spectres: These demons and not something to mess around with. They're the school bully, the ones who can kill you fast. If you get hit by a charge you'll stagger for a second. They are most vulnerable on the back.
(Cyber)Mancubus': They take good damage at close range especially with the Cyber's acid. Their projectiles are really accurate so beware. Mostly keep an eye on them and you'll have a chance.
Baron of Hells: These demons are nasty, they can deal great damage like Mancubus' and Hell Knights. These are just a bigger, badder Hell Knight reskin kinda.
Lost Souls: These enemys can really hurt you too. They are able to fly, but nothing minimal damage can't do to stop them. Just keep an eye on where they are.
Cacodemons: These demons can be deadly at close range, normally 1 Siege shot, 3 SSG shot and more can kill them fast.
Summoners: These enemys can summon any enemy (hense the name). They can normally die from some Rockets or Lock-on Burst Mod.

Ok that's all I have for now. If you liked this then if you want you can like it (I wouldn't care).
Keep the bloodbath going.