I've always struggled with pile ons as a player and I don't remember how I handled it when running games. Basically the strategy is you go after the character who has the least health/armor and does the most damage. Everyone attacks them the first turn so they can't do that damage again.
Pileons don't really work in a real world fight because you would be running into each other.
The other game mechanic touched on here is entanglement. Most roleplaying games do not have good rules for entanglement and I often befuddled the guy running the game with characters designed to use it.