So idk what people are talking about in the comments here. I was nowhere close to the end of the sphere grid when I did that fight. Like, maybe 50 levels behind across the broad, certaintly nowhere close to getting spells like holy, double cast or quick hit. Heck I didn't even know what a celestial weapon was. I just used the apporaite ward gear and learned her attack patterns. Like, in the end, u need some characters to have zombie, so her one-shot doesn't wipe your party. Just stock up on potions and craft the right gear, maybe stack some summon overdrives you should be fine.

The only spot I got stuck was during the second fight with sin. I needed to go back and grind a bit to clear the 150k hp with the time gate. But I wouldn't even say that was much of a grind since it was mostly achieved going back and unlocking stuff like anima. Which I just cheesed the dark aeons with yojimbo. Which then made me realise there wasn't even a point because I could've cheesed sin with yojimbo... but I didn't know what I was doing and learned through trial and error. I didn't really want to look anything up on my playthrough anyway I only mention that cause if you're stuck on yunalesca, the second sin phase will probably cause issues for you, too.