I'd change the inherent for sentinels to work something like this:
Give sentinels a non-damaging, non-aggroable aura that surrounds them. Have it pulse once a second.
IF the pulse hits an enemy, give the sentinel a buff to regen, recovery, resistance, defense, and recharge (any or all of these would be fine). This buff lasts for 5 seconds and is refreshed every time the pulse hits an enemy. It does not stack with multiple enemies
If the pulse hits 0 enemies, give the sentinel a +Damage +To Hit, +Range (Any or all of these would work)
Secondly allow X% of damage to pierce resistance Y% of the time.