You must get top the very bottom of the Pit of 100 Trials. Bonetail is the final enemy. He is the strongest enemy in the game. You must be very strong to fight him. I would recommend leveling u[p a lot before fighting this guy. This is what Goombella says when using her move "Tattle".
"That's Bonetail. He's the oldest brother of Hooktail. ...So old he's just bones, in fact. Wow. Now THAT'S old. His body's Max HP is 200, his Attack is 8, and his Defense is 2. He has various breaths that might confuse us or put us to sleep. When his HP gets low, he'll recover... or reanimate, as the case may be. He's probably stronger than the last boss, seriously! Let's do this right!" - Goombella
If you defeat him, he'll spit out a treasure chest. Inside is the "Return Postage Badge".