Describe the crash
Can't fight Wither. I summon him, fight 1-2 minutes and game crashes.
Did it 4 times already.

To Reproduce
Steps to reproduce the behavior:
Summon and fight Wither.

Crash Log
Last 2 crash reports:
https://paste.dimdev.org/vewatuzoqu.mccrash
https://paste.dimdev.org/vimugajidu.mccrash

Info (please complete the following information):

• Modpack Version: 2.76.2
• Server or Singleplayer: Single
• Version the world was created on: 2.76.1, but chunks are new - generated on 2.76.2

Additional context
Tried it in The Nether and Overworld both, same result.
Was able to kill it 1 time on 4th try, but all loot fell into lava, so i tried kill him again and it crashed.

Looks like Magma Monsters and Wither Skeletons are both being called by Entity.isOnSameTeam and EntityAIHurtByTarget.alertOthers, perhaps when the Wither is hit by player or vice versa, somehow crashing the game. Are you wearing baubles/other items that would cause hostile mobs to become friendly?

No, but those last 2 times I tried to fight him in creative, just for it to be easier after 2 fails, maybe that's it?

Apologies for that. There's a bug where the skeletons summoned by the wither want to retaliate when hit by friendly fire, but cannot, and crash instead. It'll be fixed in 2.77.