Edit: With the 5 stunt points, Rothas will use the Pierce Armor and Lightning Attack stunts.
Rolls
Axe attack, damage - (3d6+5, 3d6+3)
3d6+5 : (535) + 5 = 18
3d6+3 : (353) + 3 = 14
Lightning attack with axe, damage - (3d6+5, 3d6+3)
3d6+5 : (661) + 5 = 18
3d6+3 : (114) + 3 = 9