A quick guide about my way to play cassiopea , based on this amazing mage item Rod of Ages :)
this item allow you , as opposed to the classic Tear of the Goddess + Rylai's Crystal Scepter to have mana efficiency , hp and damage asap .

Greater Mark of insight : Just no other good choice for mages , give you nice damage boost at all stage of the game

Greater seal of clarity : Help you regenerate your mana when you out of blue buff , you dont really need mana regen before lvl 6 so /lvl is the way to go

Greater glyph of force : Just increase your damage , same than mp5 you dont need it that much before lvl 6

Greater Quintessence of potency : Same than glyphs , increase all your damage

Standard mage masteries , giving you :

- % spell pen to increase your damage (rised to 49% with void staff)

- 9% cdr , always help for your ultimate CD

- mana regeneration and increased experience for early laning

- buff duration for your lovely blue buff

- movement speed and flash cd

My skill priority is :
I max Noxious Blast first , your main harassment tool and an important part of your burst damage

After i max the second part of your burst damage : Twin Fang

Miasma is more an utility spell and the slow is enought early on with 1 rank .

: allow you to get kills easier with your full combo ie your stun , the 2 dots Twin Fang spam and ignite

: allow you to get in range of your ultimate to kill somebody , but you will most likely use it as an escape tool

Tear of the Goddess is the safiest way to solve mana issue , because of low cd spells and the passive cassio can charge pretty fast .

But you just dont need it !
with the mana and mana regeneration from Rod of Ages combined to your loved blue buff you have no mana issues , outside of blue buff you have enought mp5 by runes/masteries and mana pool to handle it

You can kill blue buff really easily with cassio , just dot him with Miasma and Noxious Blast , use your Twin Fang 3 time and then repeat until it die

Your core item is simple :

After these 2 items you simply go for "standard" AP items depending of the game :

: Mandatory item for AP , just change the order between this and more defensive items

: more HP to help stay alive , AP to deal damage and a slow on your abilities , like will of the ancient it will only add a 15% slow to all your abilities except Twin Fang because they are AoE spells !

: greatly increase your damage output when enemy team stack magic resist

: help you stay alive when your always focused

: help with cc and/or combo champions like annie veigar or leblanc

This was a quick guide to show my way to play cassiopea , I hope it will help you and if you dare please vote for it !