First of all, thanks, glad to hear you enjoying the game so far^^
As for your question: I would say the recommend level is around 30 to 35. Most of the dungeons can be done in any order but that one is considered one of the later/ harder ones. So good job on beating her with only level 25!
She definitely has some hard hitting attacks. The first insta kill attack she uses in the first turn is more like a puzzle or gimmick. The intended solution is to use a skill that gives "Withstand Lethal Dmg" while having a high hostility on the character who uses it. Some skills that can lower her accuracy also can help a lot.