He isn't a superboss by any means. What happens is that you have the option of fighting in eight rounds. The odd-numbered rounds are fought inside of Monstro while the even-numbered rounds are fought outside in some ocean where you attack Monstro directly or with explosive barrels lain around on rafts. The even-numbered rounds also have a time limit (about 2 or 3 minutes IIRC) where you automatically go to the next round when time expires. The only exception to this is the eighth and final round where you have an unlimited amount of time to finish Monstro off.
Now what's really interesting about this is that if you play your cards right, you can end the battle prematurely and finish him off in just two rounds with Round 1 being inside of him and Round 2 fighting against him directly. For me, I hit the explosive barrels right at him, set up Seeker Mines (or Mine Squares if not playing as Aqua), Thunder Surges and had fun with Ghost Drive.