Hey guys

My name is RMYF and I'm from TW server. I'm a Platinum 2 player and a former Yasuo main till I tried Cassiopeia, when I tried Cass, I instantly want to master this champion, because of her damage and her ability to outplay people.

Pros

High damage
Really high DPS
High potential

Cons

Skill dependent
Low mobility
Hard to master
Easily countered by assassins

You can go really crazy if you have very good mechanics, but if you're not that good, I sincerely suggest you not to play Cassiopeia. If you don't land your Q then Cassiopeia is basically trash, so you need the ability to always hit that Q if you want to play Cassiopeia.

Flash A must.
Ghost This helps you to get reaaaaaaaaaally fast and so you can dodge the skill shots you need to.
Cleanse Cleanse removes enemy cc and helps you get out horrible situations.
Exhaust Use it when you're facing an Assassin like Zed or Talon.

I choose Rod of Ages because it provides mana, hp, and ap, and those are the basic elements of Cassiopeia.
I choose Morellonomicon because it gives you cdr, a bunch of ap and mana regen when you kill an enemy champion.
Archangel's Staff Think I don't have to explain this item.
Banshee's Veil Gives spell shield, cdr, and magic resist, pretty good item for every mage.
Void Staff Magic penetration against tanks, build this so you won't do literally 0 damage against tanks.
Rabadon's Deathcap Gives a lot of damage, you can kill adcs in a few seconds with this item.(2017adc lul)
Abyssal Mask You should rush this instead of Roa if you're facing a heavy burst ap champion or you're losing your lane really hard.

Because you bought Corrupting Potion you can go in front of the enemy mid laner's face and spam your e to chunk his hp. When you get lv 2, try to hit Q and go for the kill if the enemy mid laner is already 1/4 hp or half hp. Because of Cassiopeia's lack of mobility, you'll easily get ganked if you're pushing too hard. If you're pushing the wave, try to ward before you do it, because you don't wanna get ganked and die for no reason. Once you hit 6, you can have less fear for the enemy jungler as your ult is perfect for countering ganks and if you're already destroying their mid, the jungler is just another kill if he comes.

Cassiopeia is pretty hard to play, but if you play her well, you'll be unstoppable due to her high dps. Just don't forget to be cautious of ganks and try to hit your Qs. Ciao!