@Mesh @Phuture or a blend of ideas for offense...when a HP barrier's health is depleted to 0, could have it rebuke a Holy damage nuke against the attacker (e.g. at a similar level as a cleric's highest known nuke or as a proportion of the barrier).
or other such rebuke type effects in general to deliver damage or debuffs
e.g. debuff a mob and every time it attacks one of your allies for a duration, it takes holy damage etc (or just next X number of times)
so sort of a reactionary nuke or damage over time. either delivered directly as a debuff on a mob, or lingering from other cleric support effects on allies triggered when attacked
For example, make Barrier reduce incoming damage by X% (before absorb) while it's active and add a cap to how much Barrier can absorb on each swing so it becomes a mitigation tool.
this what I've previously described, in Dark Age of Camelot's terms this is what would be referred to as an "ablative absorb" effect that blocks X% incoming damage on each hit up to a total HP threshold
e.g. reduce incoming damage by 50% per hit until a total of 100 HP absorbed then spell ends
or an effect that magically functions like M&M's random shield blocking absorbs
