Only one of the two parts need to be defeated. The head is the easier part to kill, but every ten seconds it hides in the shell and becomes immune to damage until it reappears. The head attacks with a physical attack and Slime, which inflicts Slow on a character. Attacking the shell prompts it to counter with Megavolt, which will take a large chunk of HP from the entire party. Ymir can use Megavolt six times before it runs out of MP.