Cassiopeia is a GREAT caster champion, if u can play her well she could very well be one of the most OP champs in game.
Start your early harrasing with Q in the laning phase hopefully in mid where cassio is most effective. Care for your mana, but if possible try to complete the Q - E - E combo, that could very easy get your opponent down to 40 or - 50% health. In lvl 6 hit with your ulti for a stun then make the Q - E - E combo over and over again. If you have time make the W too but it does not damage very much and is best for farming a minion wave or slowing down and opponent.

runes are ofc very important. I would prefer ability power, mana regen and magic pen. These are very effective for your early game harrasment and if your lucky, your opponent will get too pissed and make a mistake in order for you to take em down, or they will simply rage quit xD

not much to tell about the masteries these are very important too ofc but i dont see that u should pick anything else than what i did for cassio.

the most important items are first the rylai's crytal scepter. This is very effective as it gives you a bunch of survivability and ability power too, but the most important thing is the 35% slowing on your twin fang. This item is above all the most important for cassio. Next is rabadons wich most caster pick for the massive ability power. Then i would reccomend some defence f.ex zonyas hourglass as it gives ability power too, and banshees veil as and anit-caster item. The reason u should have some def items is that u have to get close in order for your ulti to hit. If it does not hit correctly you've ****ed up the teamfight and you class is VERY difficult because its hard to hit with the poisons in the heat of battle. And if u dont hit with poison your worse than ****...

i pick Q as a starter and lvl it twice until lvl 6 together with twin fang. I do this for the very effective early game harrasment. After lvl 6 u should go primarily for twin fang as it is your highest damaging ability with a VERY low cd if u hit with Q or W.

I pick flash and ignite cuz i think its the best for cassio. f.ex ignite is very good combined with a high lvl poison when ur opponet flee with 100 hp is easily finishes them off. exhaust can be used too if your 1v1 in a lane its very good but again i pref iginite. Flash is a must have for cassio cuz u need to be able to get away if your ulti miss, or to chase and opponent.

Pros:
very high damage is u hit with Q or W
AoE stun
****ING op harrasment early game


cons:
difficult to play
if u do not hit with spells ur useless
ulti got very low range for a caster

YOU can be the one to judge how the teamfight will end. IF you hit with your ulti on at least 2 champs ur team can easily focus them both down as u got one slowed with W and the other one will hopefully die in the stun. Your ulti IS the most important spell for cassio and what makes her uniqe compared to other champs.

farming with ***io is very important. U can use either Q or W for farming. I usually proc Q on the caster minions 2 times and a W on the melee. Together with some last hit auto attacks u can easily farm a wave of minions very early. U just gotta prioritize if u want to harras or farm early game. I pref a little of both. Anoy the **** out of your mid opponent and get some minions at the same time.