Game:Monstro

Monstro makes his debut as a boss in Kingdom Hearts Birth by Sleep Final Mix, having previously appeared as a "world" in Kingdom Hearts and Kingdom Hearts: Chain of Memories. He is fought within the Monster of the Deep battle in the Mirage Arena.

Strategy
The battle begins inside Monstro's belly, where the player will be faced with a squad of Unversed which must be defeated in order to get Monstro to sneeze.

Once outside, a three-minute timer will start counting down, in which the player 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 arenas, 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 player to get close and unleash a Shotlock onto the whale, 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 player again, and the player will have to fight their way through the Unversed to get out again, although each time is more difficult than the previous, due to the presence of Monstro's stomach acid and less fighting ground. If the player is swallowed but manages to escape four times, the next battle with Monstro will be the final round, and thus with no timer.

With one following the above strategy, Monstro should fall with ease. The timer also adds pressure to the battle, so the player must be patient in order to win.

Development
Interestingly, 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 one of the Titans appear, but as they'd already included the Ice Colossus, they opted for an alternate boss similar in size, resulting in Monstro appearing instead.