I used Scatter Mine to instantly kill Mooshroom Monstrosity.
Go to "???" level, rush to boss room.
Put down scatter mines in the middle of the arena.
And the boss instantly dies when it appears.
My power is 139, while I'm doing apocalypse+9 which recommended 153 power.
Maybe the animation of the boss health bar filling up is not an animation, but it really fills up quickly when it appears. This way when it spawned without much health it was blown up by the mines and died.