Hmm, you had issues with her? I did the exact same thing--and at the start of the level I was getting completely owned by Nitori (took me about half a dozen tries to beat her), and despite being at half mana and unable to switch out Marisa due to TP issues, I still managed to beat
with a pretty decent buffer at the end, just by flooding her with area effect spells. She spent almost every single turn just resummoning her reinforcements.
It was pretty much a game of "how many times can I use Royal Flare, Knockout In Three Steps, Master Spark, Asteroid Belt, Little Legion, etc, in the shortest period of time".