Farming on cassio is very straight forward. You want to manage your mana well before you can get blue buff. Your main objective is to steal enemy blue wraith from their jungle as much as possible. Once you've gotten blue, you can constantly push your lane with 2 Nox Blasts. After you clear your lane, you have the option of farming, your wratihs, their wraiths, your wolves or roaming to get a kill. Do not roam unless you know what you're doing. Don't roam unless you're confident that you'll be able to get a kill. Once you've gotten blue, cassio gets significantly powerful. Try to always steal the enemey wraiths when it is up, however do not put yourself at risk of dying and losing blue just for 1 wraith camp. By farming your wraiths, wolves and their wraiths constantly while pushing your lane up... You'll find yourself with 50 creep kills above the opponent within a few minutes. Farming correctly is essential.