I stun once as soon as I get it, then twice... then I wait for dreadstorm to almost finish casting and stun again (it will always cast storm after 50%). That gives you almost an extra 3 seconds of dps time. If it starts casting dreadpool a whm will fluid aura as an emergency move (with lowered acc that's not 100%), but it shouldn't get to that point. Dreadstorm IS stunnable. Then again I haven't done Levi since I got my tank mount a long time ago so things might have changed.