Charm Wave is also a great option. Places charm on all, defeat one and she summons a new clone who will NOT have the debuff, meaning one of the two WITH DEBUFF are the real one. If the first one you target is taking a while to defeat do NOT change targets, you might have found the real one.
Update: i defeated two hours after commenting this, I think it was pure luck, and somehow, Dark perpetua was quite easy for me, I even got it on the first try, the bad thing is that my game It had a bug, and now every time I try to load it it sends me to the beginning of the game, that is, when the goddess asks your name , anyway, I have a long vacation to play everything again until Ar delusia, which I think is the last updated place in the game, if I'm not mistaken.
