Just beat the turtle again as a Hard Mage at level 7 - VERY easy fight. I had 45 mana so that meant 4 x Ghost Blade (460 damage) with the troops defending, and then move in for the kill. Lost maybe 10 percent of my stacks. Shortly after this I got Magic Pole Axe which would have been even better at the second level.
You could win without the spell too. I was surprised as to how easy this fight was the first time around.
