I don't even use Stoneskin when I level. I have my VIT at 40, and blue mobs don't do enough damage to warrant taking the time to cast it. Green mobs just break through stoneskin in a hit or two, so it barely helps. And I have enough HP that I can easily keep myself alive with Cures. Maybe once we get Protect it will be different -- adding a big def boost to Stoneskin's damage absorption may make a big deal. That, and leveling shield might help, but so far my shield has leveled veeery slowly.