In case you continue to have trouble beating the fight, here are some tips that will enable you to win even without the Omega Potion:
Wave 1: Just use regular attacks, very easy.
Wave 2- Slippery Slimes: Just use regular attacks, and Marsha's Zapper.
^ doing this should leave you with plenty of Mana to heal and use spells/specials in the next two waves:
EDIT: I'm gonna remove the second half of this, let folk decide how to approach it themselves :)