If you know he's going for MB, it means he probably isn't using shear. If you can get a look at him in the open, that'll help your cause. Shear = not MBing typically.
I'd try to win the stealth battle. Inviso around, search for spot effect, run away, come back a few seconds later with spot effect now and take aim. Unless he's got a search on you too, he won't be able to cast without giving you a heads up that he's searched and seen you.
If you know he's not using shear, you can use BB. The frostbite route Leilond mentioned would be a terrific ambush opener too regardless of shear. AOEs > MMs