Against Kassadin I'd say you want to punish him with ranged auto attacks when he gets in close for last hits, forcing him to farm with Q, and then when he does use Q to farm you have a decent window of time (~8 seconds) to be aggressive and harass. Also remember that his Q has travel time, so unless he ults in to you first and casts it in melee, you should have time to fire off a couple spells to make any trades he makes with you even, or in your favor if you manage to land a CC in tandem with his Q so he can't capitalize off silence.