Here is a boss idea: Some sort of water creature (like a hydra).
The arena is a mix of shallow/deep water, there are several buttons around the edge of the arena, each button has a lever behind it, which you can attack.
You can attack the boss, but you cannot kill it with attacks or spells and the boss cannot attack you.
The arena has a timer, and if you haven't beaten the boss, when the timer is up, the arena is flooded and everyone dies.
You beat the boss by flipping every one of the levers, which creates a tidal wave that knocks the boss out fo the arena onto a bunch of pointy rocks, which kills it (the heros have to get out of the way or the tidal wave, there is a 5 second delay on the way), any hero caught in the wave meets the same fate.
But, to make it hard, the hydra automatically steps on the buttons by the lever, which flips the level back and you have to flip it again.
The hydra has an assortment of spells (all of which stun, no actual damage), some of which you can dodge, others you cannot.