Issues I see for whm are lack of dmg buff and lack of mitigation. Both of these are hard to include while maintaining the flavor of the class and would only be reasonable to add as part of a large scale change to classes. In such a situation, my suggestions would be:
group dps buff that scales with the targets' current hp, probably 5% when they are at full health
group buff that triggers a heal the next time the targets take damage, damage healed would be proportional to the size of the attack and could save them if they took more than their full hp