This idea is simple. There are mods to play together but not against one another. 
Here's how it would work. When you press "fight", you are sent to a menu with every enemy in the game. You can press on any enemy to see all their attacks. Only exception, giga queen, plus dark enemies. Rest is fine. Only kris can do this, because the others have spells. Different attacks, depending on their difficulty and damage would have different tp costs: Ruddin diamond cutter: free. Orange omega, 100%.
Act and spare are removed. You just deplete health.
Magic modifications:
Healing spells: Normal
Rude buster: Deals damage. Starts Gerson minigame. Winner takes damage. The person who initiated it however, is guaranteed to halve the opponents tp gain for 2 turns.
Scythmare: Bullet patterns will deal 20% bonus damage, reducing by 5% per turn. No limit to its usage, stacks additively, 10% max reduction per turn.
Pacify: Has a 20% chance to "tire" the enemy, reducing the speed of their soul.
I might add new spells
Both bullet patterns happen simultaneously, however if yours ends first, you must wait for your opponents to finish.
I have no coding experience, and don't even have a pc, so I won't actually make this, but it's cool to talk about.