You can wait longer between the 1st and 2nd stun. The 1st stun's timing is critical because you cannot stun the move once he starts casting. But the 2nd stun is more lax. That's where you can make up some time. You can wait until the cast of the move that he does after the 1st stun (Dreadwash?) is to 50%, then do the 2nd, 3 sec, stun. (I dont recommend waiting any longer on his cast bar because it takes a little bit of time for your stun to actually go off).