Guardian's Angel with max Cleric Abjuration + level 20 only shields 533 HP. That's not really viable for hell mode when 1 hit can take that away. The shield strength should be at least 1k range at maximum IMO. You could lower the % chance for Vicar's Wrath to occur from 22% down to 12-15% to compensate. Or if you have some sort of idea about changing the "shield" mechanics entirely, maybe something like a % damage reduction for 5-6 secs, which would also make level 20 bonuses associated with that a little more useful.