I remember starting a thread about this same problem a little while ago. As I recall, some of the suggestions there were maneuvers before you go in, with each tag or invoke letting you cast a spell as normal. Alternately, taking mental consequences you could tag for the same purpose.
Or, someone else suggested, you 'bank' the mental stress going in--if Harry takes a 1-stress mental hit, that means later he can cash it in for an evocation that would normally cost him that shift. That way, it's basically regular casting, only you pay the stress up front before the fight.