Let me start off by saying that riot was defiantly right about Cassiopeia's difficulty being one of the hardest of all the champs. Her skills are extremely hard to aim. With that being said, I went 19-3-11 in my first game using her (my team was tankless...).

In my first three games of playing this build, there was not a single champion I was afraid of. In three different games, I faced off against a fed Tryndamere, a fed Jax, and a fed Miss Fortune. I won every one on one fight I was in, BUT the ult was required for the God likes. After the farming phase, I was able to one on three and walk away with a kill or two. And I'm going to tell you how to be as venomous as I was.

Pros

-Excellent Farmer
-Great 1 on 1 fighter
-Amazing Harasser
-With a Ryali's can out run most champs
-Outstanding ganker

Cons

-HAS INSANE MANA ISSUES!!!!!!!!! (even with her passive :/)
-Ult is hard to stun with very short range(no biggie it slows for 60% otherwise :D)
-Melee DPS can be scary, since you have to be pretty close to hit with your "e"(but this works well with your ult :P)
-Stunning nukers (IE. Annie Brand Veigar) are hard to Mid against

Marks

So we all know what penetration is... it allows you to get in there and really do some damage. The more penetration you have the more damage you do. Thats why I go with Greater Mark of Magic Penetration.

Seals

Who doesn't like to spam skills that have a .5 sec CD? I KNOW I DO!!! thats why I rock Greater Seal of Scaling Mana Regeneration. Otherwise, I have mana issues all game.

Glyphs

Upon playing Cass toward the end of the game you will notice that you kinda peak, normally all casters are like this, so lets add some more late game damage on top of the .5 sec CD nuke with Greater Glyph of Scaling Ability Power. Alternatively if you find yourself getting beat on by casters you should probably switch out the AP for some magic res. I recommend Greater Glyph of Magic Resist. You may need the magic res early game. As we can give you more with your build later.

Quintessences

Early game you are pretty squishy so lets give you some steak and potatoes, and add about 75 more health with Greater Quintessence of Health.

I strongly recommend taking either Ghost or Flash to help pull off your ult, along with Ignite, Clarity, or Teleport. Honestly if I have to explain to you why or what these do I would go nuts X.x but I will put them in order from best to least for you:

Ghost, Ignite, Flash, Clarity, Teleport, Exhaust, Cleanse, Clairvoyance, Heal, Smite, Revive, Rally, Fortify

Right off the bat, you should get a Meki Pendant, a Health Potion, and a Mana Potion. You might be thinking why would I get a mana potion if I'm getting mana regeneration. Well once you actually play the way I'm telling you to play, which BTW is spam your spells till your out of mana, you will find this mana potion is more useful then two health potions.

Once you have a little more then one thousand gold you should back(normally i would tell you to stay but the sooner you get the tear the better). Buy a Tear of the Goddess, Boots, and about 3 Mana Potions. By this time you should be level 6 or higher go gank a lane, before heading back to mid.

After you have saved up another two thousand you should back, if not forced to do so already, and buy Sorcerer's Shoes and a Giant's Belt. At this point you can put down those nasty mana potions, because you are strong enough to take that blue buff on your own.

Once you have farmed and kill your way to about another two thousand gold head on back and get your Rylai's Crystal Scepter. This is where the game gets fun. You remember that .5 sec CD I was talking about? Now that you can slow them (without popping your ult), you can spam your "e" and get a lot more hits in before the poison runs out.

After you gain another two thousand gold, head on back and get your Archangel's Staff. Since you have been spamming spells all game you should already be at the maximum mana from the tear.

Once you get the gold buy a Rod of Ages and then a Rabadon's Deathcap.

Now this is where you pick your item.... If their AP is feed get a Banshee's Veil. If you are being focused get a Guardian Angel. If their AD is feed get a Thornmail. If they have stacked magic res or the tank is the problem buy a Void Staff. If i could tell you to buy one item every time I would, but you should pick from those four. I normally go with a Void Staff, because I tend to think that I have enough health from my Rod of Ages and my Rylai's Crystal Scepter.

Once you hit level 4, farming is a breeze! All you have to do is Miasma the biggest amount of mobs that you can, then spam Twin Fang and Noxious Blast on the poisoned ones. Once you hit level nine you should be able to Miasma and Twin Fang the casters once and they should die. Unless you pick up spell vamp, I would stay out of the jungle for the most part unless you are killing golem or going to gank their jungler.

As you farm, if the enemy champion is standing next to the casters start spamming Noxious Blast at him but make it look like you are aiming for the minions, if you are able to hit him start pounding him with Twin Fang. Also most champs that go mid like to run right at you for some stupid reason, so while you farm place Miasma in between you two if they run threw it start Twin Fanging them. Once you have your ult the best way to use it, is to gank from the brush, or when you are fighting in the jungle, because its simple either they are walking at you or that are not. Personally, I like to start with my ult, because I'm can normally nuke someone for about 2k in 2 secs just from my Twin Fang!

Basically, if you can supply Cass with all the mana she wants she will give you the damage output you desire! I honestly have never liked a champion more then her during that "first game" as them. I hope you can take a bite out of the other team with my guide and feel free to post your scores! :D