Honestly, I think that I might give this one to Ice King. This is X784 Gray, who has much less experience and unique abilities than his other keys, and while he does have a variety of different spells he can use, as well as a variety of weapons that he can conjure out of ice, I still think Ice King has greater advantages.
Ice King has greater mobility thanks to his flight, which would allow him to fire multiple of his ice lightning bolts from afar and make it harder for Gray to remove his helmet. Even if he did remove his helmet, Ice King could resort to his Fridjitsu. Since Finn and Jake were able to use this fighting style, it shows that people don't need to have natural magic affinity to ice to use Fridjitsu, so I think Ice King could use Fridjitsu even if he lost his crown. KingTempest mentioned beard flakes, but he never uses that ability in-battle, and he didn't even know that his beard flakes could induce sickness, so I don't see him using that against Gray. However, KingTempest did mention invisibility, so yea Ice King could turn invisible and land some surprise attacks on Gray. However, the biggest advantage that Ice King has is his Demonic Wishing Eye. This magic item allows the user to make an endless supply of clones equal in strength to the user. Though it does come at a cost, Ice King might use it if he's desperate enough, and with so many clones, there's no way Gray could defeat so many or find which clone to seal with his Iced Shell. All this, combined with the higher AP advantage, makes me think Ice King would win this more times than not.