Sorry about the weird explanation, basically, the Will Skull is an item that can heal you quite a decent amount, and only takes about two seconds. It doesn't use a lot of your magic, so you can basically just attack while tanking hits, and then use the skull to infinitely heal and easily beat any boss.
How it feels to fight anything in Shovel Knight Specter of Torment with the Will Skull (🟢⚫️⚫️ > 🟢🟢🟢) item:
Sorry about the weird explanation, basically, the Will Skull is an item that can heal you quite a decent amount, and only takes about two seconds. It doesn't use a lot of your magic, so you can basically just attack while tanking hits, and then use the skull to infinitely heal and easily beat any boss.