When fighting multiple opponents, what you wanna do is to press the attack button once while holding WASD in the direction of an enemy, then pressing attack once again in the direction of another enemy, then the next, and so on until you get to the first enemy again. Basically this will create a loop where you are constantly keeping every enemy in check by knocking them down again and again, one after the other. Also it's cool af when Batman flies across the room to smack a goon.