This minigame is usually referred to as SSM

Super Smash Mobs is a minigame available on the Mineplex server. It involves choosing a class, which is a certain mob, and you then receive special items and abilities relating to your mob class. You get 3 lives, and once you lose them all, you are out of the game.

For a far more detailed guide to this game, see the SSM page on the Mineplex wiki.