Bobob and Dumm

Both move fast so its tricky to keep them back. They also change lanes after each knockback. After one is killed, the remaining boss transforms and gains either frost or fire immunity that the dead boss had. Each transformed boss counts as a different enemy kill on the Enemy Unlock screen. Do not portal the combined one to the front as it triggers an AOE freeze that lasts longer than the stun on boss.

If you can manage to kill both bosses at the same time with a really strong spell, there will be no second phase to the fight.

Bobob = Dumm = Combined =