The spell enervation from the Advanced Player's Guide does persistent negative damage. Under the rules for stopping persistent damage, it says you have to do some action that would plausibly allow you to stop whatever is damaging you (such as putting out flames). What would count as this for negative damage? I'm thinking maybe each time the target gets magically healed (or affected with any other magic that uses positive energy), they should get a dc 10 flat check to stop the damage. Does that sound good?
That sounds reasonable.
At the end of each turn, after you take the persistent damage (no matter the type of the damage) you get a free flat check to remove the persistent damage. That should still apply in the case of Enervation.
If you take additional action during your turn, you can get an additional opportunity to remove the persistent damage. The rules give general guidelines for how to rule how that helps. Usually it costs two actions, and may require a skill check. The benefit is somewhere between reducing the DC of the flat check, or automatically ending the condition.
So for Enervation, your idea of using positive energy spells to qualify for the additional opportunity to remove the damage sounds really cool. In comparison: it isn't costing additional actions. But it is costing a spell. However, it is probably a spell that was going to be used on that character anyway. So the cost might be a bit too low in that case. Maybe add an additional action to the spell cost in order to add the additional persistent damage removal option?
As breithauptclan pointed out, you already have a DC 15 flat check to end any persistent damage (rolled at the end of each of your round).
I would allow the following additional solutions:
- Using a positive energy effect to directly interfere with the persistent negative damage instead of its normal effect (healing the target, usually). I would treat that as a counteract attempt.
- Using the residual positive energy to help the target shake off the persistent damage. The positive energy effect gets its normal results (heals the target) and the target gets an immediate DC 15 flat check to end the persistent damage.
OR
- Using the residual positive energy to help the target shake off the persistent damage. The positive energy effect gets its normal results (heals the target) and the target's end-of-round flat check is reduced (either by a fixed number, or based on the level of the positive effect).
In other words, I wouldn't allow the positive effect to both give an additional flat check AND reduce the DC.
Based on the rules you've cited, I'd suggest using Religion, Occultism, or Nature with a DC equal to what it would be for a medicine check to try and treat a mundane type of persistent damage. I'd make the DC one level of difficulty lower if the creature was subject to a positive energy effect within the last turn.
I think I'd experiment with automatically giving the DC reduction if they are healed via positive energy from a spell equal to or higher level than the enervation. It does fall in line with the likes of jumping into a river to extinguish flames or wash away acid, but as others have mentioned, the healing spell still does grant tangible benefits. If it's OP, you can alway go back on it later if you present it as a temporary ruling. Under no condition though would I consider, say, a level 1 heal spell to auto reduce a level 7 enervation though