Do note that you don't have to block against an attacker's attack. Instead of trying to stop a master evoker's main blast, do a Veil to block his perception instead, like Molly often does.
Generally speaking attacks could be as high as +10, easy. A perception roll not only is usually much lower because it is a secondary skill and lacks focus/specialization bonus but it usually can't be boosted through aspect invokation-most people don't have aspects for being very perceptive.