The info screen for Monstro's Belly Brawl.
Monstro's Belly Brawl is a one-time only minigame in Chain of Memories and Re:Chain of Memories, although it is not truly named within the former. It takes place in Monstro's Room of Truth and requires Sora to defeat enough Shadows within the time limit of two minutes to fill up a meter, which also slowly drains on its own, followed by defeating the remaining Heartless. Failing to do so will eject him from the room, forcing him to try again. Succeeding will cause Sora to be sneezed out by Monstro, which counts as completing the world.
Because the only enemies are Shadows it is recommended to use a deck with cards that have a value no lower than three. It is also not needed to use sleights, as you can get by while relying on normal attacks and magic, as it is possible you simply do not have sleights that are suitable for this fight depending on when you visit Monstro. If you do use sleights it is recommended to try those that are both fast and able to hit multiple targets at once. The enemy card effect Attack Haste from the Card Soldiers can prove useful, as is Guard Armor's Wide Attack effect if you do not have the former. Soldier and Bandit cards can also provide an edge if you have them.