This is my first guide so pardon my lack of pictures and colors.


Cassiopeia is one of the more underrated champions, but when played right she can be a definite AP carry

Pros
Great AOE damage
Low CD
Spamming Champ
Constant damage if you land poison

Cons
Requires the ability to land skill shots
No incredible burst damage like other AP carries (besides ult)

Flash is a must for Cassiopeia for escaping, chasing, and landing surprise ultimates.
Ignite is preference because of its extra damage. (People say it becomes useless in late-game but I have to disagree. At level 18, Ignite does a total of 50+20*18 = 410 damage. 410 true damage to a carry is a significant bulk of damage.

Teleport is a great skill for Cassiopeia if your teams lacks map movement. Teleport allows Cassiopeia to clear out a massive amount of creeps due to to her AOE. Since Cassiopeia is not the fastest champion, teleport allows her to enter teamfights quickly after laning or clearing out minions

The other summoners skills seem lackluster when compared to Flash, Ignite, and teleport.

But if a certain summoner skill is your thing feel free to use it. (This does not include Smite, Rally, Fortify, and Heal since Cassiopeia is not a jungler, a support, or a tank)

The basic skill sequence is QWEEE....EEE

The truth is their is no real combo for Cassiopeia. To get her damage started all you need to do is land your poison from your Q or W and land your E as much as possible while the enemy is poisoned. Since your E auto-refreshes if the other champion is poisoned. This also works with Teemo's poison.

A successful Cassiopeia Ult is when you have a W underneath the enemy champion and you get them stunned. After you get them stunned, you E and Q the enemy as fast as you can. (Smart Casting helps)

Cassiopeia's potential depends on her poisons so land your Q and W for maximum damage and kills.

[*] Doran's Ring at start for a bit AP, mana regen, and health
[*] Tear of Goddess next for constant harass and poking during laning phase and before teamfights.
[*] Haunting Guise after boots because it gives Spell Penetration, health, and AP.
[*] Why Boots of Swiftness rather than Sorcerer's Shoes? This is because Cassiopeia relies on landing her Q and/or W to enable her sustained damage from her E. The extra speed allows me to land my Q/place my W more often and thus allowing me to provide constant damage from my E. If you do not go Swiftness and rather Sorcerer's Shoes, you can skip Haunting Guise in your build and go directly to your Rylai's.
[*] Rylai's provides a slow which allows you to land your E because your E's range is not as big as your Q. It also gives AP and Health. When building Rylai's, start with either Blasting Wand or Giant's belt depending on your situation. If the other team has alot of poke and you are not doing well in your game go for Giant's belt for survivability. Otherwise go for the Blasting Wand.
[*] Next item is Rabadon's. Rabadon's is the best item for AP. This item will dramatically increase your damage potential in team fights. If they have alot of AD or huge nukes such as Veigar's ult or Karthus' ult, you may opt for Zhonya's Hourglass since it gives armor and 3 second stasis.
[*] Archangel's By this time you should have maxed out your Tear of Goddess so it only makes sense to complete it with Archangel's for some extra AP.
[*] This late in the game teams have probably completed some magic resist items, so it is time for you to get some spell penetration. Usually I go for Abyssal Scepter for the magic resist and its aura. If the other team is stacking heavily on Magic resist I will definitely go for Void Staff. Also if the my damage is lacking I will switch my boots for Sorcerer's although I rarely find this necessary.

http://clgaming.net/board/board39-reign-of-gaming/board40-blogs/board56-beyond-the-battle/2373-battle-1-void-staff-vs-abyssal-scepter/

^ this website shows how Abyssal Scepter and Void Staff match up with each other (check it out)

[*] Finally your last item. This item depends solely on what you need. If you are beasting the other team and your game hasn't ended yet go for more AP such as Zhonya's if you didn't take it before. If their AD is targeting you like mad and your team can't get rid of them get Thornmail or a Guardian Angel. You are tanky enough where you will be able to soak up some damage for the Thornmail to work. If you need more magic resist for for Banshee's Veil or Force of Nature depending on the other team. Go Banshee's if their AP carries require a certain skill for their whole combo to work (Brand).

Nothing much to say here other than stay back and deal damage because of the range of your abilities. Only get close when you are about to use your ult on multiple targets. Don't get caught.

Your role in teamfights is to place your AOE on as many people as possible and soften them. Once all cc has been used by the other team, go in and clean up.

Kite and poke your enemies before and during teamfights
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These are some ranked games with Cassiopeia. Unfortunately the games I lost were with leavers