Rebuke is fine vs large packs of monsters, but horrible for bosses.
Idea: have two modes of added damage, and a rule on which to take.
Mode 1: current text (20 flat damage per hit while channeling)
Mode 2: 25% of all damage prevented while channeling, but only if the hits are all from a single enemy.
In other words, if a boss is the only thing hitting you (usually just 1-3 times), that will trigger mode 2 because no other mobs are around. But, if there are lots of monsters hitting you, then it uses Mode 1 which is current behavior.
Note: 25% is a placeholder value, but I think that a 900 health character who stands directly in a Large/Primary Boss Ability (with no defenses) would get 1-shot, so figure that’s 1000 damage, and 25% would be only 250 damage, so I think its fairly on par with 12 regular monster hits @ 20 damage each (240).