Try to hit the enemy and get gold from minions. Keep your enemy at the distance. To deal ap damage you need to make your enemy have stacks from W. More stacks = more damage, max is 3 stacks. W pasive put stacks on enemy when you aa him, You can put stacks in other way too, imo better way, hit someone with R, that will stun him and put 3 stacks on him realy fast, than pres W, Hold for 4sec Q and than relase. Try to use W very often.

fightning assasins are realy hard for you to win.