Ability which can only be used once in a battle

There are different ways to achieve this, e.g. having a special Consumable status value as use cost for the ability or using requirements and setting a variable.

If you don't want to clutter your project with unneeded status values for special abilities, I'd recommend using (object) variables:
- the ability uses Requirements, checking for a bool variable (e.g. object variable on the combatant)
- the ability's battle events set that variable on the user
- the battle start (or battle end) event removes/resets the variable on the player group