Well, you're simply not gonna get 'sevenfold vengeance'. The game simply doesn't have a mechanic for anything like that...it'll almost certainly be their own attack directly, or a flat value of some sort.
Hmmm. Well, Wards actually already do exactly this, so what you seem to be looking for is effectively a character who's Warded. That's...difficult to do.
Personally, I'd be inclined to grab Sponsored Magic (Mark of Cain) which might have other abilities, but the important one is Warding at Evocation Speeds and Methods. You then put all your Item Slots into, well, the Mark, a defensive item that makes a potent Ward with plenty of uses per session (this is best done as an item, since their use is Reflexive). Making the 'item' impossible to remove is at least another -1 Refresh...or might be covered by giving up conventional Channeling entirely.
Any way you make this is going to be very jury-rigged, really. Barring Death Curses, the system is really not built for this kind of reflexive ability to hurt somebody. They're certainly all going to be at least -4 Refresh like this one.
A better solution might be to just grab Marked By Power, grab an appropriate Aspect, and have the 'sevenfold vengeance' thing be a perfectly mundane threat by God as opposed to something mystical, and which he will carry out in his own good time and does nothing to help the marked guy. Like diplomatic immunity in a way, since it's just a bad idea to hurt him, not impossible.