This is the final boss battle. It is story-related and cannot be missed. It occurs when you return to the Ice King in the Ice Kingdom late in the game.
At level 10 with 10,000 HP, Uncle Gumbald may initially look scary, but the fight is relatively easy. My level 9 characters were more than enough for the task. Just make sure your characters have enough experience and a fair amount of healing potions. Gumbald's special attacks can be nasty.
I used Jake to protect Finn and BMO, while BMO was charging the other characters' abilities. I attacked the pirate minions first before focusing on the boss, and used a few resistance orbs along the way.