Regen over Stoneskin on the OT if possible. Stoneskin on a ~10k HP warrior will prevent ~1800 damage, and that requires a warrior cooldown. Regen on that same war will likely tick for over 2k on him unless he's topped off, and the whole point of the OT on Levi is that you can afford to not top him off for large portions of time because he doesn't take a lot of damage unless adds are up. Regen, like Stoneskin, doesn't give stacks, but it saves 80 mp per cast. It also benefits from Divine Seal or Convalesence.