This was my first persona game and I didn't read too much into the whole fusion system. I just messed around and tried to get high level personas by triangle fusing. Now that I'm on January 8th, I decided to see what kinds of ultimate fusions I can do, and it looks like I messed many of them up by not doing every quest from Elizabeth. My main personas that I use are Surt (Ragnarok), Odin (thunder reign), Loki (bufudyne and garudyne), Melchizedek (mediarahan and physical attacks) and Raphael (buffs), all in the low to mid 60s. My party and I are all lvl 67 right now. I was planning on leveling up my party to about lvl 75 before I took on the final bosses of the game. If i just continue to level up these personae, will they be powerful enough to finish the game? If personae with certain skills are needed, which ones would be the easiest to obtain?

no if you are around 75 you should be fine. the finals boss weakness changes so you just have to adapt

You'll be fine, I got the best ending and I used Surt for a lot of the heavy lifting

I'm pretty sure I got through the entire final battle spamming Mind Charged Thunder Reigns. Took maybe ten minutes. You should be fine. If levels are your concern, then unlock Monad and grind for a half hour, you'll hit 80 in no time. Hell, if you can beat the reaper then you probably have no worries any way.