Hm... the Lapin fight was... extremely easy, I simply used my own Melissa and a support Rena, rapidly built up Melissa's Magia through Accel chains and then nuked Lapin to high heavens, took exactly one attempt, seemed like the most obvious, uncomplicated route to take and Lapin never got even close to building up her Magia.
I don't recall it being explained anywhere that the Tart Magica girls got a significant powerup for that battle(unless it was in the story, which I was skipping due to not having read/watched the first one), which meant it took me possibly a dozen battles to finish her off(luckily, autobattle was available for the second and subsequent rounds).