mercy strike is a nice have, but not absolute necesary. Stoneskin on the other Hand is a big Bonus, you have only Flash to waste mana on otherwise and in situations where your healer are struggling or even when you just have a Little bit of time at Hand you can make it easier for them. Stone Skin might seem at first like it is not that much, but it can replace something between 1-2 vita/phisyks I would highly recomend to get this one at least.