On my first game, I think I was using Zidane, Dagger, Vivi and Steiner during the Memoria, although I switched Vivi to Eiko during the final battle. My levels were probably around 60-70 for the first four, and maybe around 50 for Eiko.

On my second game, I used Zidane, Dagger and two out of Vivi, Freya and Steiner. Vivi has Doomsday (does a lot of damage and heals everyone), Steiner has Climhazzard and Shock, and Freya has Dragon's Crest. Freya's especially good in the final battle because Dragon's Crest will always do 9999 damage even if the boss has cast Protect and Shell, or even if you've been hit by status ailments. On this 2nd game, Zidane and Dagger were on level 99, the others were on levels 70-80+.