I'm going to go with the agreed-upon rosters of Seymour, Golbez, Sephiroth vs. Cielo, Athos. Agahnim. I'm also assuming we're only using abilities from these six mercs. Therefore:
Athos looks at Golbez's admittedly shoddy magic and realizes he master all that stuff during his apprentice years
Athos takes along everything except Aeroela and Fortify
Cielo asks Seymour, "Why transform? It's not all about power...."
Cielo also brings Phys Resist, Life Surge, Resolution
Done.
--
You beat yourself up with your past. Don't blame yourself, blame the world. Blame God. Blame me.