Standard build for cassio, kinda agressive play style, however, you need to be sure to get in your poison skillshots.
Playstyle is agressive, since cassio is very strong at early levels, you gotta be very agressive to win, but make sure to not overextend, if you get behind, 9 out of 10 games it will be hard to get back. When you push out you opponent (In mid lane obviously) you can try to Q and W over his wraiths and steal them over the wall, will make you stronger but also make the enemy jungler abit behind aswell, and he will fall behind in levels making it easier for you to not be ganked. Try to communicate with YOUR jungler, cause you will need the blue buff as fast as possible, however, Cassio doesnt take too much mana, but too make sure you can harass and zone them as fast as possible.
The runes i've chosen is kinda standard AP mid runes, you got the early Magic pen, and also some health regeneration, which makes you able to stay in lane untill you can buy the next item you need, plus some AP quins for the early strong AP, plus abit defensive magic resist cause the enemy AP in mid lane is gonna hurt you aswell.
The masteries are aswell normal AP to get as much AP as possible, and with buff duration increase for the blue buff obviously.
I always start with boots and three health potions, i think this is most viable to try and harass at level one and up, after that i go two doran's rings (or three)for the health mostly, after that you need the spell vamp, since when you get blue, you can just mass out your spells and gain hp from farming basicly, which make you strong as hell. After, you need some more health fast, to make you strong. Then just go whatever the games tells you to be basicly, if you die too easy, buy some more hp, Warmogs armor before other AP items, or if you dominate, you buy more AP.
When fighting or harassing, DONT use your E before you hit your Q and W, your poisons, cause otherwise, the E (twin fang) doesnt get low cooldown enough to harass/kill your enemy fast enough. Also, in team fights with your Ultimate, make sure to stand in front of your enemys to make them stunned, this is a really good initiate for team fights aswell as it is a save for you, basicly if you get three enemys against you, you just ulti them, poison, and then Twin fang (E) and you should be able to burst 2 down while they are still stunned.
I take Ignite and Flash as my summoner spells, and its the most standard AP mid summoner spells. Flash, like a extra save if you get in trouble aswell as it could be a flash to kill someone early and late ofc, but mostly my enemys get away with about 10% hp, then i flash after and burst em down. Aswell ignite for those moments when your enemy is very low and you can flash and ignite them and you get the kill. Ill flash for a kill any day, some peoples disagree, but i would really do it, a kill, early will give you such a advantage.
Cassiopeia is viable for soloq, but you do need abit of communication and teamplay, cause yes, Cassiopeia does alot of the teamfight damage, and your role in the team fights, is basicly to just hit the ultimate on atleast 2 peoples and then you should have a fairly big advantage in the team fight, and then just get out as many spells as possible (poisons first) and then, as i see it, that my role in the teamfights.
Farming is kinda autopushing, at level 4, Q and W makes the ranged creeps die so you can basicly put your poisons down and move away if you have miss in the lane and you can help your team, while your enemy looses farm in the mid aswell.
This is my first build i ever done in LoL, i have been playing this game for about a year or so now, i feel i can help people out, and explain why i build this and this.