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Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
This is my shot at a Cass build...
Cassiopeia does a lot of damage. As with the case of most high damage champs she's also squishy. Our goal is to maximize damage output while maintaining optimal survivability. Since the caster friendly offense masteries are at the top of the tree we can pick up armor pen, ability power, and upgrade cripple. The rest go into defense, we have to survive to get kills.
The rune makeup is similar to the masteries. Cass is super squishy and people tend to mark her as easy to pick off, hp is our deterrent. Since marks and seals offer the higher hp boosts pick up nine of each of those and nine glyphs of potency and three quints of potency as they offer the higher ap.
Since Cass tends to take a lot of damage I use upgraded heal. One cast usually buys enough time to miasma and run or blas and twin fang for a kill. Cripple is as close as it gets to cheating. Use it on any melee hero and they can only watch you tear into their hp.
I pick up miasma first for the slow. It can be used defensively to run away or offensively to prevent fleeing, it is super useful if you're the type to run to the bushes for first blood. After that twin fang needs to be maxed first, then noxious blast. The ideal combo is blast->fang->fang->miasma->fang->fang. It isn't usually idea to chase since after that miasma has a longish cooldown, this is the time I go back for items and hp/mana. If my opponent notices and decides to stay a teammate can go get an easy gank. My favorite time to use petrifying gaze is from the bushes right before the enemy steps in to see you, drop a blast and fang whoever has the lowest hp then miasma and run. It can be interrupted and you have to be very close to use it so I've found that it's best in team fights when running away to stun as many as possible.
For items I start with amplifying tome and a hp potion. We get +139 hp to start from mastery and runes and +66 ap. After 330 gold I go back for Kage's, it pays for itself after 11 minutes and by the end of the match pays for your shoes, but the important thing is ap not the gold.
About this time we start to notice how much mana a snake can burn so a tear of the goddess is next. With the rate she casts spells it developes a deep mana pool in a hurry. Get a blasting wand next(gotta keep ap up throughout) and then depending on how much trouble the other team is giving you a giants belt or tome. Then finish the rylai's crystal scepter. Now the blast->fang->fang combo has a 105% chance to slow it's target.
Rylai's grants a pretty big hp boost as well as ap and a slow so at this point Cass is pretty well survivable. The goddess tear should be full or almost full by now so we get a blasting wand and archangel's staff.
At this point there is a decision to make. At this point most casters won't be a problem as fast as we can put out damage and tanks we can stack damage until some support comes. The only thing cass has to fear is someone doing more damage to her than she does to it so depending on the enemy champs a frozen heart for high melee dps or a banshee's veil for spell dps. If the other team's dps can't touch you then get a zonyas.
A void staff is never out of the question if you see someone stacking magic resist.
Don't forget an oracle's elixir if you are fighting twitch or eve. They are fairly cheap, last until you die, also twitch and eve are easy kills with cass if you can see them.