Stoneskin can be a very useful tool and extremely efficient on the high health war tanks, and even good for pally's but I feel a little less efficient. I seemed to struggle a little bit in the 3 levels prior to getting SoS, and then got a little nervous right before you get its traited upgrade that doubles the mana regen potency.

Like the others mentioned, regen is your friend, melee dps typically will take constant damage, or ranged take some moderate hits of 30% or better use a regen, it becomes extremely efficient and does a good job. Sometimes I will regen the target, and stay attentive to them as much as the tank ready to switch over for a fast cure II if I see that player get hit again, but typically the regen heals them to full.