As you can see in the video below, some wierd bug kept happening where the boss and the adds resetted for some reason during the fight. We solved this problem by trying to stack everyone on Garr and managed to kill him but only after 6 tries.
There were other bugs in MC with annihilators spawning lava spawns and firelords spawning an absurd amount of spawns.
Here's the video of Garr bug:
https://www.youtube.com/watch?v=rf7IcK-CtpU