Beat the game and then load your cleared save 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.
To easily max out your weapon and strength, beat the game to make a yellow cow 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 attack it with any weapon. It is very powerful and will take a long time to kill which allows you to max out his weapon rank and strength.
To easily gain money, after getting the Thief Suit, go to the castle and find a mimic. The easiest one is in a room in the hallway where the monkey was. Continue to go in and out opening the mimic to get money. Do not kill the mimic; just hit him a few times until he runs away so that you can open him again.