With him mentioning polymorph as being the summon that takes the most mana specifically, it kinda makes me wonder if he's gonna summon another powerful polymorph to finish this out. With the only other non-human I can recall he would know really would be Aria/The End (his maid). That one would probably take all the rest of his mana, if he could even manage it, lmao. But would make for a good climax.
Dunno how tense we're supposed to feel from the other two shadowy conspirators, though, when the lady is someone he already trounced when he was a child. I guess she could be better prepared this time, but still seems like it'd be easy since he's been powered up since then.