Cassiopeia is a great nuke champ, Some would say her Damage output late game is similar to Viegar if used correctly.. Some tell me that Cassiopeia is useless because her ult is nearly impossible to land on good players.. Well im here to say.. its easy as hell.. just gotta know when to jump in and when to run away..

Usually i build her with AP per level runes because of her deadliness with her Q can be devastating late game, you will find yourself just mashing Q all night as if u were Karthus..
If you don't have the runes necessary to complete the build but wanna be just as deadly invest in some mana Regen per 5, either that or Build a chalice, which i wouldn't really recommend unless the other team is caster heavy..

Dorans ring and mana pot starting items, during early game play avoid bursting all your mana right away and wait to Q(Noxious Blast) minions when they are at Half HP then E(Twin Fang) for the kill.
Usually after boots i build Catalyst for Rod of ages but sometimes its better to get soul stealer if your kill streaks are going well, chances are though you aren't as deadly as you want to be 15 minutes into the game unless the other team has feeders.. don't rush to get the kills let them come to you..
Play Cassi extremely passive till your tank up then wait for the party to Flash/Ult stun for the kills..

Late game
If your team is running with more than one caster encourage them to get Will of the ancients with you so you get a nice 40% Spell vamp as opposed to only 20..
Otherwise go with more spell Pen on the Void Staff

You will come to realize that Cassi is more like Fiddle in the sense that shes EXTREMELY Squishy, and only does well when not being targeted.. but with enough ap, your noxious blast will Destroy team fights and the other team will see Enemy double kill every few minutes..



Try to control your Teams Golem Buff since Cassiopeia's late game depends on her constantly casting spells, Sure it might seem unfair to everyone else but look at the scoreboard after you win or lose.. you should have the most kills for your team.. should be sitting at 800AP+ and be able to constantly spam Q for 900-1000 damage

Cassi is an amazing champ in the right hands, since all her abilities are pretty much skill shots, its recommended that you don't play her if u cant lead with Q attack