Only advice to offer ya is get stronk. Leveling up in the last area is super easy and you can jump up a lot of levels in maybe an hour or so. Also just fuse personas, get new ones, and just go all out with the possibilities. And just buy back ones in the velvet room you want to keep. Being stronk is very important and wish ya all the luck, and if you have beaten it by time you see this post, congrats!

Took me a minute to find out who you were talking about. Follow Bona's advice. I think I went and fused for Vishnu for this and the final boss.

Don't forget to farm money so you can get really good Personas in your next playthrough!

At that point all you can do is grind. Lavenza gives you free heals, so don't worry about saving SP.