You'll get this right away when you defeat the last boss. He's not hard, but you do need to make sure your ready to block his attacks.
I'm not quite convinced there is a magical combo of costumes that are super effective against him since this game is fairly strait forward and simple but I got it on my first try with the robot, the pumpkin and the star trooper one. The extra damage and dodge stamps are awesome against him since if the dodge kicks in on the one he attacks it stops the splash damage as well.
Alternate good costumes to use are the unicorn that can revive allies with the build-up attack and the statue of liberty that has an awesome boost/heal build-up attack that affects all 3 of the kids.