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Successfully complete the game. Load your cleared game file to have the Cherry Sword, Invader Him, Unlock The Real Me, Wild Nside items in your inventory. Additionally, the three records can be played on the sound system at Rizo Island.
Once you complete the game, a very powerful yellow cow will appear in Caladoxia Ruins. Unlock the doors to its room and lure it to the L-shaped staircase where you leave that part of the ruins; it will not be able to attack you now. Then, climb halfway down and go into Battle mode, and Terry will automatically keep attacking it with any weapon. It will take a very long time to kill and his weapon rank for that weapon and strength will max out after a while.
Once you get your Thief Suit, go to the castle and find one of the mimics. The best one is in a room in the hallway where the monkey was. Keep going in and out opening the mimic to get more money. Make sure not to kill the mimic. Just hit him a few times until he runs away so that you can open him.