When playing cass play extreemly passive. if you try to gank a champion before you have at least misuma and noxious blast you will likely fail. the easiest way to gank early is push your enemy champion out of your lane. i recomend taking a lane solo, mid or top depending, then slowly dwindle thier health using noxious and misuma. forcing them to recall with spell doom for them, because of having the exp bonus from the utility tree you will level faster, also if they recall you can often get at least 2 waves of minions killed.

the abyssal cepter is often over kill but i have had many a game where people would start to stack magic resist agaist me so that i would be hitting for nearly 200 with a tick of poisens. if you have a match where it isnt needed, i would strongly recomend an extra deathcap, or some other high AP item. and stick with the void staff, either item replaced works very well.