Story: No story here. Just a typical ice vs fire matchup.
Special conditions: Heat Man, aware of his Ice weakness, is forcing Bowser to protect him. Save Celestia for last, because when she's defeated, Twilight will be afflicted with *NOOOOOOOO!*, which devastates all her stats.
Empowered Iso-8: Frozen Iso-8 (ice users only) - ice attacks provide the user with Icing Up, increasing defense. Chance to give an ally Resist Bleeding when your turn comes.
Battle Strategy: Have Elsa and Freeze Man attack Bowser first, since he's protecting Heat Man. Have Twilight repeatedly Teleport Kick Heat Man until he's down. It's stealthy, and he will fumble and go down faster than the other two. Once he's down, Twilight can target Bowser. At all costs, avoid defeating Celly before both Heat Man and Bowser, or you're already dead. Keep Celly cornered with Books That Bite and Internal Freezing.