All five units must be in a Straight Class. No two units can share a class. (Aka, no Secondary, R/S/M from base Job are the only ones usable.) Any Special Class (Ramza's Squire, Holy Knight, Engineer, Divine Knight, etc) counts as the Generic Squire class, meaning you can only use one "Special" unit in its base class or one "Squire" in your party at maximum. You can, obviously, change a unit's Straight Class between battles, but all the previous rules still apply, so if you use Agrias and have her as Holy Knight but need Mustadio to be an Engineer, Agrias much change to a different class since Engineer and Holy Knight count as the same Straight Class. Etc.
All non-Ramza units must be set to Auto Battle, and once you choose their AI at the start of the fight it cannot be changed.
That shouldn't be too difficult, but would force you to approach the game in a pretty different way than you're likely used to, since you can't use tons of OP combinations like MP Switch + Move-MP UP, and you're forced to learn to fight alongside your Auto Battle AI buddies as Ramza. New approaches to battles, and being able to get new appreciations for various classes without an excessive amount of difficulty should equal quite a bit of fun and interesting stuff.