The developers are planning to do something to balance solo content (solo raid,WBR,tel harsha/misrus) for healers, so here is a suggestion that could help both defensive and healer classes:
Add a new enraged mechanic that increases the attack value of the boss and at the same time decreases the defense value of the boss every time it accurately damages the player character (only player characters, no summons/assisters/companions) with its actions/skills.
This would allow the healers and defensive classes to stall the fight until the defense is low enough so they can deal as much damage as the offensive classes deal without causing much suffering for the offensive classes if the stacks are low enough to not matter that much initially.
My suggestion is 0.1% atk increase and 0.1% def decrease per hit the boss accurately lands on the player, capping at 50% atk boost and 50% def decrease after getting hit 500 times.