Kingdom Hearts: Chain of Memories/Riku/Monstro

From where you start, kill all of the Heartless and strike the door. Synthesize to make a value of 1 or more. In Room 2 do the same and in Room 3, do all the same except for synthesizing to make a value of 2 or more. In Room 4, clear the room of Heartless and strike the door. Synthesize to make a value of 3 or more. In Room 5, after killing all of the Heartless, strike the door. Synthesize with cards that have green borders to make a value of 5 and the Key of Beginnings.

Parasite Cage
Right off the bat, combo the Parasite Cage. Before it even has a chance to attack, you'll get the Gimmick Card. The Gimmick Card will make the platforms stop rotating and give you easier access to the Parasite Cage. Now, without the threat of falling into the acid, this boss shouldn't be any trouble at all. If it does manage to get an attack in, here's what to expect: Once you've taken care of Parasite Cage, go back to Room 2 and strike the remaining door. Synthesize to make a value of one or more. After clearing the next room of Heartless, go through the door and you'll return to Castle Oblivion.
 * Its tentacles slam down to make the platform Riku is standing on sink. Switch to a different platform or continuously jump if you hit the acid. Get to another platform as quickly as possible.
 * It slams its body into Riku. A simple attack. It can cause some bad damage, but not anything life-threatening.
 * It emits gas that, if not avoided, will hit Riku and bring his health down.