Get yourself the sword, wtih which you can capture monsters, when you kill them, you capture them. You need 10 monsters of each kind for the battle arena, and then Penance(don't know if i spelled it right) will appear (forgot where, sorry), hardest monster in the game, 10 million HP and so on, pretty cool.
Where to get the sword?If you don't know it before tonight (meaning someone else posted it, which they prolly will do, I might be on tonight, and I'll tell you then. Gotta ask a friend for ya.
Greetz,
EW.