Each character has a variety of attacks that each have different purposes. Projectiles can be used to rack up damage from afar, long reaching strikes (Donatello's staff attacks, for example) can hit enemies from outside their own attack range, and attacks with low knockback can start combos. Make sure you know the speed, range, and power of your moves before you head into ranked. You can use training mode training mode (Single Player -> Dojo -> Training) to learn each of your character's moves, or watch the official character spotlights on YouTube.

I get it, enemy near, press buttons. But sometimes, that's what they want you to do so that they can dodge and counter attack. Sometimes it is best to save your swings until you see an opportunity to attack.

When your enemy is not on stage, please do not just let them back on. Make at least some attempt to keep them from the ledge or you will have a hard time scoring KOs. Even if you find directly chasing them too risky, you can use projectiles to pester their recovery, or even set lingering traps at the ledge (Rocko's jackhammer, for example).

Slime meter is the defining game mechanic of this game; failing to use it effectively will result in defeat. There is so much you can do with slime, but at the bare minimum you should be able to amplify your recovery moves (LT + the special move of your choice) to make it easier to return to stage and use slime cancels (LT after landing any light or charged attack) to hold enemies in place for another attack.

At the end of the day, this is a game, and the whole point of a game is to have fun. If you are not having fun, take a break for a while and come back replenished and ready to go!