Phase 1 isn't too bad, Ran has a skill that AT advances someone and gives them true strike, so you can just use that and shield bash alternatingly to guarantee a stunlock. (I just didn't have enough MP to do this turn 1 without some MP healing items, but I can go grind out the starting MP passives).
I don't think killing the boss in one hit is remotely possible for me lol. I put up all the buffs I could and hero strike doesn't even take off 10% in phase 2, then he kills a party member and puts another at critical HP (or puts the entire party at critical HP with the healing debuff). I'm trying to do this right when it unlocks at level 20 so that's not even close to possible. Guess I'll just go grind out all my skills and 20 more levels or so. The fact that it warns you about the timer made me assume it was balanced for doing it right after you get Ran, when clearly it's balanced as endgame content and I'm here way too early