[FaeLenx]

Just make priest and other heal heavy classes gain a debuff that lasts a couple turns that blocks them from getting healed from other sources.

Or add a lore friendly implication to the overuse of Mercana and make every use of healing Mercana magic on the field increase the FP cost of the next healing spell by 10% for both friend and foe. You could even add an invocation to purge the counter if you're worried it'll ramp up too fast.

Or 100% interference after round 30 of any fight that reapplies every turn.