Cass is a great champ and i believe her to be one of the few champs that can completely build glass cannon. the reason is because unlike other casters, her damage is not limited to a single burst but a steady stream of DPS and like physical DPS who stack lifesteal AP DPS should stack spellvamp and go glass cannon because the spellvamp is a really nice way to keep her HP up without sacrificing damage

the down side to this is your mana. you will be mana hungry all throughout the game. tear of the goddes is a good way to go but it takes alot of effort to farm that AA and it doesn't even give enough AP for its price

the runes are strait up mana regen because you need mana. the reds can be mpen or flat AD depending on if you want to hurt more with your spells or if you want an easier time farming early game. in all honesty i really doubt that either of them will do much for you. the masteries follows any of the standard or non standard 9/0/21 sequence. the summoner spells can be any popular AD carry spells. heal is also nice too if you like the bit of healing when fighting a long fight and you dont have a support champ on your team.

max E asap. it is the primary source of damage for you and if your fast at mashing your buttons then you can cast it at the same speed that most AD carries attack at. the best way to play her is to default E to smart cast spell 3 and just have your mouse over the target and mash E after you have maxed E it is pretty flexible on what skill to max next. if your team has alot of CC then max W because your team can force them to sit in your pool if not then max Q or you can also level them at the same time because they're both good in their own way.

the core of the build lies in spell vamp. the reason is because you have a consistent stream of magic DPS (assuming you dont mess it up and forget to poison the target) just like other glass cannon AD champ who stack alot of life steal, you can stack a whole bunch of spell vamp and do crazy damage while not having to worry about your own HP. needless to say CC screws you over. the boots are tied between merc treads and boots of swiftness simply because you dont have legs to use them. or because you want speed to hunt down fleeing opponents or get away or make the most of your ghost. merc treads are pretty self explanatory. starting after you have compleated your second revolver you have a few options. one is to go strait up AP with death cap and just pump out damage like mad. second is get the third revolver and then branch from there and the third is to buy rylia's and put some CC onto your spells and the last option is to buy lich bane for the additional damage output from there you can start building giant AP one way or another all depending on what your favorite build for AP is. stack AA if that's your preferred build. anything to get you alot of AP

alot of people say that cassiopiea's ult really doesn't synergies with her kit but i'd have to disagree. the reason why is because its a harsh CC that your applying which synergies really well with your miasma which in turn combos with your twin fang. additionally its a great tool for counter initiation. a AOE nuke that stuns for 2sec is great for anything however because it only stuns while they're looking at you, it's only great for when the fight just started and all the enemy are trying to get into your face to kill you.

miasma is will slowly grow bigger so it might be wise to set up a miasma pool before a fight at a place where the other team will likely step in and punish them hard for stepping into it. as well by default miasma's cooldown is the same as its duration so assuming you get CDR you can actually have 2 miasma pools on the ground at the same time for about 2-3 seconds.

dont facecheck a bush check it with your Q because if someone is in it you'll get a speed boost and you probably shouldn't go in.