@RetromanDamage taken from Hellgate Warden and Gatekeeper spells can no longer be reflected
You may want to extend those measures to other contents. Damage reflection on mobs is exported to other types of PVE contents. Some mobs in PVE high level content can be obliterated by their own damage so easily that it kills the challenge you propose.
I think it is fair to remove the ability to reflect mob spell damage especially if the player benefits from both a reflect spell and immunity
You may want to change the way it's calculated. Mobs attacks (all of them) shouldn't be amplified by the reaver branch bonus when they are reflected. Not sure this is efficient to add another condition in your coding but I wanted to suggest this option.