Edit: gil was so kind as to make this a native feature, you just have to check "AI Controlled Player" under Combatants > Combatants > AI Settings.
Is it possible to have the whole player group, including the leader, to be AI controlled? Combatants > Combatants > AI Settings > Not Controllable says the combatant can still be controlled if he's set as the player or the first / only combatant in the player group.
I'm talking about the whole battle being fought by the AI, although at least something like the "confused" status in FF would probably work as a workaround (I'd just have everyone in the player group being confused all the time, depending on if I can set up a battle AI "Confused" that then gets applied).
Alternatively, can I set up an AI battle between two factions "Allies" and "Enemies" and set up the rest of the game acting as if the Allies are the actual Player faction? Meaning they can be equipped, have a player controlled skill tree etc.