Kingdom Hearts Chain of Memories
Kingdom Hearts Birth by Sleep Final Mix
Monstro is introduced as a boss in Kingdom Hearts Birth by Sleep Final Mix, after appearing as a world in Kingdom Hearts and Kingdom Hearts: Chain of Memories. He is fought within the "Monster of the Sea" battle in the Mirage Arena.
Monstro was not the original choice for an additional Disney Boss for Kingdom Hearts Birth by Sleep Final Mix. A Square executive originally suggested having another of the Titans appear, since they'd already used Ice Titan, but they wanted to surprise people, resulting in Monstro appearing instead.
The battle begins inside Monstro's stomach, where the protagonist will be faced with a squad of Unversed that must be defeated in order to get Monstro to sneeze the player above the surface of the ocean.
Once outside, a three-minute timer will start counting down, in which the protagonist must deal as much damage as possible to Monstro before it runs out, though it can be tricky due to Monstro's mobility and the scarce attacking ground. Monstro will occasionally slam his tail on the arena, dive across and even try inhaling the player to sneeze them out and deal damage. The main strategy of the battle is to hit Monstro as many times as possible with the explosive barrels while he is swimming slowly. Once hit enough times, Monstro will go dizzy and leave his tongue hanging out, a sign for the protagonist to get close and use a Shotlock, possibly twice if quick enough. Occasionally, Monstro will slam his upper body onto a small island and briefly leave himself open to melee attacks.
If the timer runs out, Monstro will automatically swallow the protagonist again, facing them with another squad of Unversed; each time the protagonist returns, the amount of stomach acid increases, leaving less area to fight. The final round occurs after the fourth time the protagonist escapes, and will have no timer.
A high-risk but fast method of winning is to equip five Mine-type commands, one Mega Flare command, and one Group Cure-type command (to increase medal payout). The player should run up to the center of Monstro's stomach and quickly use Mega Flare to destroy the Unversed and force themselves out onto the islands; the player should jump to the largest patch, use their Group Cure command, and may opt to use a Mine command should Monstro fly past. Eventually, Monstro will slam himself onto land, and at this point the player should expend their remaining Mines in the hopes of dealing as much damage as possible; with some luck, it is entirely possible to end the battle on the second round.
Notes and references