Hi, I'm CoolioAlt ^^ This is my Cassiopeia guide. I'm a newb as Cassi, but she's easy for me to play and I usually do pretty good as her, So I decided to make a build. Sorry if it isn;t good, but it's my first champion build.

Feel free to change these, but I went 4/0/26.

Pros (+)
+Good DPS
+Low Mana Costs
+Good at farming minion waves
+Great crowd control
+Ranged
+Hot Upper Half
Cons (-)
-Squishy
-Damage is not instant
-Horrifying Lower Half

I'm not very good with Runes, so you can take what you think is appropriate, but I took 8 Greater Magic Penetration Marks, 1 Greater Mark of Knowledge. Then 8 Greater Glyph of Potency, and 1 Glyph Of Force. For Seals, I took 8 Greater Seal of Potency, and 1 Greater Seal of Knowledge

Recommended Summoner Spells
Flash- Great for getting away, which Cassi with need to do a lot.
Ghost- If Flash fails, this is nice back-up.
Ignite- If your poison doesn't do enough, then this should help it.

Okay Summoner Spells
Exhaust- Can be used to help take someone down or to get away from them.
Cleanse- Another possible escape mechanic, or just to turn team fights in your favor.
Clarity- You'll spam a lot, so if you get low and can't recall, this works.
Clairvoyance- Use to stop ganks or spy on enemies
Teleport- If your turret is in trouble

Try not to take
Heal- If you play her nice, you won't need this.
Fortify- ...
Smite- You won't be jungling much
Rally- ...Really? Does anyone use this spell, anyways?
Revive- You won't be dying much.... I hope.

So if you manage to get everything that this guide suggests, you should be pretty OP and nearly killing everyone by spamming your skills on them. Good luck ^^