Equip Peach with the Lazy Shell Armor so she only takes 1HP of damage from anything. That way she can constantly heal when needed. Is Culex that dude from that one weird room? The guy who says he's from another world? If so...
SPOILER
Culex is a crossover character from Final Fantasy (remember, Square made Mario RPG). It looks similar to Astos from FF1, and it has the Four Crystals from the first five games. Also, after you win, the classic Final Fantasy Victory Fanfare plays.