When we're talking about an everything or nothing skill like an instant kill, I think it should always be situational and rely heavily on a good setup. This is also a server balanced entirely around group play and players who group up for combat should reap the most rewards. Of course, at some point, you're going to be killing them with raw damage anyway if you can debuff them that much.
If AAs need a new capstone, maybe something appropriate can be suggested. But looking at the state of other PrCs, several of which have even less impactful abilities than this, without having anything close to a permanent +5 on ranged attacks, I just don't see why it needs buffing. AAs are already amazing at killing low fortitude enemies like casters, avoiding all the damage of reflection shields and piercing premonition. This ability doesn't need to stand in for some deficit they're having in that role, and I don't think I want to see them capable of instantly slaying anyone with a primary fortitude save either.