I'm not even going to pretend i'm better than average with Cass, but I can damn well do decent
//1:// Personally I play Cass (and alot of other mages in her category) with a /focus/ on AP but nabbing extra HP whenever I can
which leads me to //2:// (and //3://) since Cass' passive lifts some weight off her Mana I usually go for a Catalyst the Protector into a Rod of Ages
Cassiopeia generally needs to be close enough to land her E and R spells so having a nice chunk of HP and Regen isn't a bad idea, as well as topping her off with some more Mana
I try to stay away from CD and Mana Regen since they're pretty much wasted on her, so most likely my 2nd real item is a Liandry's Torment
(I don't count boots as a 'real item') and I add on Sorc Boots and a Voidstaff for more Magic Pen
Radabon's doesn't even need to be mentioned, and for the last item it really depends-
--you can choose to add in a Archangel Staff early (if you really want to),
i'm liking the updated Twin Shadows alot more
or a good old fashioned Hourglass for safety (and/or delaying the inevitable)
//4:// the way I see it, Cass' job is to be as annoying as possible, spitting out as much aoe poison on as many people as you can, trying to win the next teamfight before it even begins
more seriously (if I can be asked) is Zone Control, making the enemies go where you want or else step in a pissy puddle of poison
/during/ teamfights you need to spread your poison on everyone but mainly focusing on spamming your E on the most important target while also making sure they're still poisoned so you don't put it on cooldown
all of Cass' teamfights hinge on proper use of her ultimate
I like to flash into it like a Sona ult, but if you fuck up you probably won't live to regret it
alternatively you can use it as a counter-engage to stop everyone in their tracks when they go to jump on you
//5:// i hate discussing runes, to each his own, i just have ap and pen
//6:// Cassiopeia can only trade when her target is Poisoned so you have to go Q>E>W for levels 1/2/3 then maxing E>Q>W
//8:// the rest of this is 90% based on the status of the game you're currently in
if you have a clear advantage (say, a Fizz who has no idea what he's doing)
then you should poison him, spam your E (poison lasts for about 3 E casts)
then back off and repeat
if you're getting your ass handed to you (say, a Fizz who knows what he's doing)
then farm as much as you can and poison him whenever he tries to cs near your turret
Cass' Q gives her a speed boost, and her W slows enemies
you can roam if your situation allows it, but everything depends on if you can reliably land poison
