Nice. Good use of everything they had developed up until then, their teamwork, even their individual quirks, and then a callback right to the very first chapter where Wasabi offered Aki a note for "Beginner's login bonus. Help for saving. Guts." which got this entire tale started. Agreed that this was a surprisingly fulfilling battle.
Also, so Marom was indeed the sister, though changed by their experimentation with light/dark power and the enormous passage of time just like her brother. Nice to see a bit of extra peeks at what lead to the current situation.