How would this feat interact with a warrior who's using Heightened Invisibility? If the enemy isn't attacking him, but is attacking his ally who's next to him would it work? Or does enemy have to target you atleast once per round?
I would say as long as the encounter lasts and the foe knows you're there somewhere, he takes the damage. The feat only says he must be in combat with you, it doesn't say he has to attack you.