I can't imagine someone having troubles beating the boss, but if you are I have a gold combination of Costumes and Battle Stamps.
Costume 1: Robot with the High Damage Counter-Attack stamp
Costume 2: Space Trooper with the High Damage on Critical Hits stamp
Costume 3: Statue of Liberty with the normal Counter-Attack stamp
The pro's of this setup is that 2 of your party members can counter-attack ( after a successful block ) which gives almost as much damage as a regular attack and the other one has a high chance of doing massive damage. When your Super Attack is loaded you use them all 3 in a row. The robot missiles do great damage and so does the space trooper's special attack. The statue of Liberty's special attack heals all party members. This is a gold combination, I defeated the boss with all 3 party members intact.
You could also use the Vampire or the Pumpkin head instead of the Robot or Space Trooper. As long as you select 2 costumes with high damage special attacks, and 1 costume with healing abilities. The unicorn also heals, but only 1 party member while the Statue of Liberty heals all 3.
Basically this is a High damage setup with healing capabilities.
Note: Great tip by Quarantane
that The vampire is a good substitute for the Statue of Liberty since he does great damage and has healing abilities.