I've had a little time to think, and I think I've arrived at.. a compromise

1) Remove Reprisal from WAR and PLD. Make this DRK exclusive again.

2) Reduce effectiveness of Shake it Off. Base shield 4% with each buff consumed adding 4% up to a maximum of 12% HP

etc) It might be possible to boost the effectiveness of Reprisal, 15%? Not sure, there's obviously a point where incoming damage would give one an advantage over the other, but it shouldn't be incredibly difficult to balance their effective mitigation.