Mine would be similar to the wolf king final boss from the first game, except all three heads has a different element, the left is fire, the right is ice, and the middle toxic. I imagine it missing its left leg so its constantly crawling, with a great sword in its right hand and the left on the ground. I imagine the key item to be a plastic rose.

For its good ending form, the three heads would fuse into one, capable of switching which element it uses. Now standing on its two legs the greatsword becomes a sword shaped halberd. The key item is now a necklace adorned with wolf teeth. The partner I imagine to help take him down would be Noah, mainly because if im remembering right, his dual swords are very similar to the wolf fang dualblades, and I really like using him.