Updates:
Cassiopeia is a difficult champion to play and relies heavily on your skill as a player. Unfortunately if you miss even one skill shot it could spell doom for you. Take your time when learning Cassiopeia and pay attention to how the enemy moves and attempts to evade your attacks. Cassiopeia is a damage of time champion and excellent at harassing, so don't be over zealous and try to tower dive at level two. The goal of this build is to nuke hard via magic penetration. The amount of magic penetration in this build makes even tanks melt fairly fast.

For the sake of understanding I will refer to the build that focuses on the utility tree as Squishy and the build that focuses on the defensive tree as Survival. The first half of this guide will focus on Squishy and the 2nd half on Survival

Twin Fang Twin Fang: cast animation sped up.
Petrifying Gaze Petrifying Gaze:
The animation speed up on Twin Fang is actually really strong. I am not so sure how fast it is yet, but any speed up suggests less cooldown. Before the real cooldown was roughly .7 or .8 seconds because of travel time, but now I feel it will be .6 to .7. If your target is always poisoned this is roughly a 14% damage increase.

The buff on Petrifying Gaze seems overpowered to me, but that could just be me. The damage and bug fix is fine, but the range now is ridiculous. It is the same range as a Noxious Blast.

Conclusion: Riot wants to put emphasis on landing Twin Fangs while the target is poisoned, which makes the reward for skill higher. The buff on Petrifying Gaze was long needed and now seems more versatile thereby making it more practical in an offensive situation as well as better in a defensive situation; however, the 850 range might need to go down to 775 or 800 in order to be more balanced. The range part is merely my own personal opinion.

Pros:
Great team fighter
Nonstop harassment early game
Serious damage throughout the whole game
Good farmer
AP carry

Cons:
Being able to use her Q skill is a must
Squishy

Flash: Cassiopeia doesn't have that good of an escape. She can use her W skill to slow and run away and Q them to get more movement speed, but in tough situations Flash is what saves lives.


Ignite: Ignite synergizes very well with Cassiopeia's move set and can add that little bit of damage you could be short on because of mana. I tend to take it in Survival because I seem to be more up in their face and therefore in range of using it.

Masteries: Standard caster.

Runes: Standard caster runes with the exception of two movement speed quints. Allows for better positioning which is vital on Cassiopeia.

In order to play Cassiopeia effectively there are three things you must be able to do well: read the flow of battle, understand positioning, and be aware of the mini-map.

Flow of Battle: When I refer to the flow of battle I am referring to how people will move, predicting. This skill is especially vital during the laning phase as that is when people are trying to dodge Noxious Blast the most. Many of my first Noxious Blasts aren't really about hitting the enemy, but putting myself in tune with how they move. At this point the enemy has control of the flow of battle and I am merely observing. After observing I can then usually take control of the flow of battle by landing nearly every single Noxious Blast. To a small degree the flow of battle is involved in chasing, but not as much since people move in such a linear fashion when running. I don't consider it a necessary skill to have when team fighting since getting people poisoned isn't a hard job; throwing two AoE poisons in the middle of a clump has a very high success rate.

Positioning:Cassiopeia operates like any squishy ranged carry, she needs to have a team that can protect her well. Theoretically if you are good enough you could kite the enemy you are trying to kill infinitely without receiving damage, but this skill is not useful if that enemy is a tank chasing you out of the team fight thereby stopping you from wrecking their squishies. To prevent this scenario from happening get a team composition that has at least some kind of protection/distraction.
Landing Cassiopeia's ultimate is tricky because you don't want to go in that far to hit enemies and you also don't want to miss. The solution is use it as a follow up to another move like Amumu's or simply don't use it until you see a point where you can hit a lot of enemies. Her ultimate is tricky because it takes both skills, positioning and flow of battle, to use effectively.

Mini-map:This skill mostly applies to the laning phase. You must be aware of who is jungling, and who is missing. Another thing to ask yourself is, is my ultimate up? Having your ultimate could be the difference between your death or getting a double kill off of their fail gank.

General: Ult when she ults and win.

General: Kassadin and Leblanc are very threatening champions because of the fact that they can compromise position so easily and dish out a huge amount of damage. In addition they both possess silence which often spells doom for any mage, but more so Cassiopeia as she needs to cast more spells than an average caster in order to kill them.

Strategy: Kassadin is awful early game and you should be zoning him hard. Take as much lane control as you get because after 6 he will start to assert himself. Leblanc is the opposite. Stay the hell away from her and out farm her early game. Leblanc is awful late game.

Kassadin is the natural enemy of Cassiopeia. He is tough to deal with and takes a lot of skill/luck to beat.

TL;DR: Either ban Kassadin or pick Cassiopeia last.

General: She nerfed so hard. Faceroll her.

R>E>Q>W. If you land a Q use the movement speed bonus to go up and start Twin Fanging. Low level of Q lets you keep high stacks with minimal cost.


Q:Noxious Blast
Must be able to hit with this skill. Short cooldown is good. Damage good too.


E:Twin Fang
If you hit Q go up and start using Twin Fang. You can use up to 3 before the poison ends (travel time makes it 3 instead of 6).

Cassiopeia loves blue buff. Get it when you can. Late game it is a must.

Cassiopeia is a serious AP carry, but takes extreme skill to make sure everyone is poisoned. She also requires a good team for protection or else she will melt. Similar to Ashe she is very reliant on positioning. Hit what you can as hard as you can. Don't try to get the squishies in the back, leave that to the people with dash skills.

Bad results on Cassiopeia are usually attributed to bad players. She is by no means awful and can achieve amazing results if you are good with skill shots.