I was doing a lot of theory crafting on if it was possible to Solo 1 Turn the Lavenza and Twin boss fights even with unusable moves.
I gave my Pixie Will of the Sword so it starts concentrated and triples the dmg output instead of 2.5x then stacked Magic Ability with Almighty Boost + Amp then added Tyrants Mind. In order to truly make it a single turn I added the 3 auto passives creating a Super Pixie just like Elizabeth has in P3.
This Megidolaon usually does 9999 dmg but since the twins massively resist it even when triple concentrated and attack is up on Merciless we can JUST get the true one turn kill with it.
I've theory crafted some stuff for Lavenza but I'm not sure its possible. She has a damage limiter meaning you can only skip halfway into her next phase. I was using a calculator in my Level 1 Solo Lavenza battle and everytime I used Riot gun to skip phase 2 no matter what damage variation I had in the first 10 turns of the fight she always had a total of exactly 12,500hp lost. However a few moves like Hassou Tobi which multi-hit can bypass this limiter which is what you do to kill her in 2 turns