Currently, Cassiopeia is my favorite AP carry. She dominates every single AP carry. Her harass is insane. and when you trade with her they will lose every time. If you learn to play her correctly, I don't see anyone being able to win mid against you.

Pros

Outstanding damage
Ult can stun whole team
Actually synergizes with other champions(cooldown resets on Twin Fang if the target is poisoned by Teemo or Singed
Can fit any type of team comp.

Cons

No escapes.(Thats why we take Flash)
Depends on skillshots
Targeted quickly.
Mana hungry(needs blue)

Greater Quintessence of Ability Power
Too strong of an early game to pass up
Greater Mark of Magic Penetration Magic Pen late game is too strong to pass up. Must have
greater seal of replenishment For the early game spamming. If you dont think your jungler will give you blue then feel free to grab Greater Seal of Scaling Mana Regeneration
Greater Glyph of Scaling Ability Power Gives more AP than the flat AP at level 6.

Works great with her skill set. It allows you to harass while not burning up too much mana.

Wonderful spell for poking your opponent. If you land this on the enemy champion, it also gives you a nice speed increase. So if your getting ganked, try to land Noxious Blast and then Miasma to slow, to get away safely.

This spell is for those of you that can't land Noxious Blast. If you throw this directly on the enemy champion, you have 2 seconds of them being slowed to land Noxious Blast (as it will be much easier) and then start to Twin Fang them.

This is what makes Cass so strong. However, this can also cost you to lose your team the fight. If you Twin Fang them and they aren't poisoned, then you have 4 seconds where you can basically do nothing. Insure they're poisoned by throwing Miasma on them first.

Knowing how to use this spell determines how good of a cass player you are. Try to position yourself so that you can hit at least 3 people, preferably the carries. In laning phase if your getting dove, as soon as the person tanking the turret is taking shots, throw Petrifying Gaze on them, Noxious Blast then Ignite and Miasma and start to Twin Fang them. They may be squishy enough to get a kill.

Boots and 3 health potions have much more sustain than that [image= doran's ring size=25]. Also makes it a lot easier to dodge skill shots.
Too strong of an early game.
I usually go magic pen shoes. However if your not doing well, Merc treads are perfect. You'll still do a ton of damage late game. Another time i always get Merc treads is if they have two AP champs.
This is where is depends on your matchup. Usually Rylai's is a better choice overall. It gives your Miasma even more of a slow, and even makes them go 15% slower when you Twin Fang. Yes 15. For some reason riot decided some stupid permaslow thing was too strong, so they changed Twin Fang with rylai's to a 15% slow.
If you rush Rabadons, Your damage output will be ridiculous but you wont have as much health, therefore you die easier. It's whatever you like better

You can either ward here, or on either one of the bushes. Depending on whichever bush you ward, hug that side at all times. That way if the jungle comes to that side, you can see him and go to your turret. If he comes from the other side, just run to your warded side and you should be fine.

First, grab your boots and 3 health pots and then go leash for your jungler. After thats done, proceed to lane and immediately start harrassing your opponent with Noxious Blast. Each time you hit, try to Twin Fang him 2 times and then retreat back. Once you hit 6, life becomes fun. If you know you can kill your opponent with your ult but he wont get close enough, flash then ult. Its incredibly quick if you do it right and your opponent will have no time to turn around and avoid then stun. When you stun him, Noxious Blast, Ignite, then Twin Fang to death.

If your lane is pushed and the other person is freezing the lane, feel free to gank other lanes, preferably bot. If you angle yourself correctly, you should be able to get them both stuns, assuring at least one kill.

Cassiopeia is amazing for team fights. If you can hit atleast 3 people with Petrifying Gaze you've did your job. If the only person your able to poison is the tank, then do it. You take down tanks easily, especially after you get Void Staff. If the other team has that stupid Olaf that runs through your team cc just for you, thats fine. Just kite him until your team reacts.

] When playing against ahri, just try to dodge her skillshots. If she cant hit you with her Charm, her whole combo goes down tremendously.
Medium

] Harass every time she goes up for a last hit. At 6, if she jumps at you throw down Miasma where your standing and she'll be slowed. Then Noxious Blast her and start to Twin Fang
Medium

Dodge that stupid Flash Frost and you'll be set. She'll probably go for late game but your still the stronger champ late game.
Easy

Annie is different. Level up q>e so you can push the lane, causing her to have her farm hurt or lose that stun proc.
Easy

Brand does work. That is if he can land Sear and get a stun. DONT allow him to do that. That is the only way he can beat you. If you have Blaze on you, get behind minions so he can't hit you with Sear.
Medium

Fizz. I hate him so much. He can bounce to you. While avoiding all your poisons. Ult you and you die. Extremely gay. So you should level q>e and just throw Noxious Blast and Miasma on the caster minions. then throw Noxious Blast on the other minions and just hug the turrent.
HARD

Galio. Another champ that is rough for cass. His harass beats yours and your poisons do nothing to his tankyness. level q>e and just farm. Don't try to trade with him. Rough lane.
HARD

Gragas is a hard matchup if they have a damage dealing jungler. At 6 he'll try to ult you where your closer to him they the jungle with gang-bang you. Not fun. So ward your lane. Hug that side and you should be set.
Medium

Trolololol. Heimer is just easy. He'll push your turret so level q>e Just don't get hit by CH-2 Electron Storm Grenade and you'll be fine
Easy

He's just going to farm against you. Sadly you can do nothing about it really. Try to zone out as much as possible. If he gets close enough Noxious Blast. Make him pay for farm
Easy

Harass extremely hard before 6. After he's 6 just harass and went he goes in for you hit him harder on his way out.
Medium

Lets see. Kat is hard. When she tries to Shunpo in Miasma behind you and start to Twin Fang and Noxious Blast At 6 she may be aggresive so when she shunpo in and use Death Lotus just walk out And the next time she goes in ult her and do work.
Hard

Pretty much like Kassadin except she hurts alot more. Pick up an early Negatron Cloak and you'll do fine
Medium

Dodge her Light Binding and you'll be fine. Yes she can harass early with Lucent Singularity but it doesn't do too much. If your getting harassed too hard, feel free to pick up Hextech Revolver early.
Easy

Get an early Quicksilver Sash and you will dominate lane. When a minion has Malefic Visions it will transfer to you when you die so don't stand near it.
Easy

Mordekaiser. Everyone says that Cass counters his shield. I however disagree. Morde does tons of damage and i find it hard to break his sheild anyway. Level q>e and just farm it out.
Medium

Easy lane. No doubt. Her Dark Binding is incredibly easy to avoid. And her spell shield basically only absorbs the poison. Only thing i have to say is don't ult while her Black Shield is up. If it is, the stun/slow from your ult will not affect her.
Easy

Her ball does not do that much. She is an easy champion to play against and will mostly like to farm. If she does farm, then level q>e and harass all day. Try to avoid ball and you'll be fine.
Easy

Ryze has no comparison to you in laning phase. At level 2, he cannot stop you. Land your Noxious Blast and you'll destroy the lane.
Easy

Sion is really just a silly champion. DO NOT allow him to bubble and stun you. If he bubbles, back off and Noxious Blast him from a distance.
Easy

Swain is not to bad a matchup. Just avoid that annoying Nevermove and you should be set.
Medium

Level q>e and farm. His Noxian Diplomacy and cutthroat will destroy you. Just farm.
Medium

If he has a gold card and is running at you, back away, make him hit a minion with it, and then continue to dominate lane.
Easy

Veigar is too easy. Run Cleanse, and after 6, once he thinks he can combo you dead, just cleanse, and you kill him with your ult. GG
Easy

Viktor is dumb. He wont win lane, but he also wont let you snowball. At 6, be careful about his Chaos Storm. It silences you and does a bit of damage. Play normally and if no lanes feed, you put out double damage late game and you should win.
Medium

Vlad isnt too hard. Hes going to try to Transfusion you, but as hes running at you, Noxious Blast him and he should back off and use it on a minion. If he Sanguine Pool while you are Twin Fanging him, then back off.
Medium

He's just going to outrange you. Level q>e and try to harass. You should never get close enough to twin fang. When he has magic chains on you, avoid the Arcanopulse and you'll do fine.
Easy

Avoid his bomb and you'll do just fine. Watch out for his across the map Mega Inferno Bomb. If you need to just flash out.
Easy

When he comes up to you to try to put a bomb on you, just go for him, and he'll regret having came close to you. No problem
Easy