Beat all Stage cave bosses (you do not need to beat them in caves they just need to be bosses that can spawn in the cave floor)
the cave bosses are
-Chub
-C.H.A.D.
-Gurdy
-Gurdy Jr.
-Mega Fatty
-Mega Maw
-Peep
-Dark One
-Pestilence
-The Fallen
-The Headless Horseman
-The Stain
-The Frail
-The Forsaken
-Big Horn
-Rag Mega
-Fistula
-Bumbino