New Champion Overview: Kai'sa
Updated for 8.6
So with the 8.5 patch we were all introduced to a new champion, Kai'sa. Kai'sa is a strong champion using mainly physical damage, with a kit made to be usable in almost any situation. Kai'sa needs to focus on three key stats to get the most out of her kit: Damage, Ability Power, and Attack Speed. Kai'sa is a late-game burst damager who is weak to 1v2 but can pick off almost any champ solo from mid-game on. This guide will explain how to play a reliable Kai'sa, but of course it will not be the only way. Have fun experimenting with her and find your own playstyle to get the most out of your League experience.
the first thing to consider in any game is your summoner spells. for a midlaner with burst and mobility there's always the classic Flash Ignite , but there are some alternatives you may wish to consider:
Heal is a surprisingly utile spell, making up for the lack of survivability you will often have before you can get your builds going. This will also enable you to focus on your output, as well as providing basic protection from early ganks. On top of that having heal is great to use as bait if you're facing an enemy 1v1, backing away at low health while your cooldowns reset, then popping heal and diving right back in with a Supercharger(E)-Icathian Rain(Q) combo, or even a Killer Instinct (R) if you haven't already used it. Free kills anyone?
Your next option to consider is 
Teleport, the classic choice for outer lanes to quickly get back into the fray. If you're taking Kai'sa to the bot lane as ADC this is a must-grab spell. I like this option on Kai'sa as it enables your team to feed you into an early carry. You can use it in all the classic ways (TP to turret or one of your lane minions to jump back to the action with minimal time loss, or to a warded bush for surprise assaults and teamfights, and to support your fellow laners with a quick burst before returning to your lane with enough time to reset cooldowns), but with Kai'sa's increased mobility and quick damage this becomes even more dangerous. Definitely a great option to consider.
Considering how reliant Kai'sa is on staying mobile to deal damage at range, 
Cleanse is a must-grab against enemy CC, unless you plan on building from 
Quicksilver Sash (QSS). This can be a great option, and 
Mercurial Scimitar is a great situational item. However this forces you to choose between QSS or early damage, and a well planned enemy defense can take advantage of this no matter which way you go. Of course taking QSS AND Cleanse means you'll never have to worry about pesky CC in teamfights. Jump in after your tank with burst, use your E to get away as best you can, Cleanse away any CC you get hit with and jump back out while your abilities are on cooldown. Your teammates can finish the job efficiently and your team will appreciate your opponents' lack of skills.
Ghost and 
Exhaust are debatably useful to secure your mobility advantage, however I personally believe it won't be necessary. 
Barrier is the only other semi-viable option, but the shield itself isn't particularly strong, is limited to a 2 second duration, and won't apply to teammates. For these reasons I recommend Heal over Barrier.
Another key skill in League is to know when not to take the champion you want to play. Champions that counter you will counter you hard, and will often focus on making your life as miserable as possible. For example any champion that has a good "pounce" effect to close the gap between you and him/her is dangerous. She is very weak to ganks and shouldn't be played against mobile junglers with burst and durability like 
Warwick, 
Shyvana or 
Nidalee. CC is dangerous too as a squishy champ, so QSS is a must buy early if you get counter-picked with an enemy 
Leona (otherwise consider Cleanse as one of your summoner spells). Always consider team comp when choosing your champs. having a hard CC support is great too, as your Void Seeker (E) is a long-range skillshot that applies your Plasma stacks, which are required to launch at an enemy with your Killer Instinct (R). Synergy wins games, so be sure to keep up your skills with other champs as well.
Now that you've gotten to the game you're gonna want to plan out which order you want to focus on your abilities. Since each of your abilities focuses on a different stat to boost into its secondary form, it's your best bet to focus on the stat of the ability you want to get the most out of early. your Q is going to be your main minion farmer, enabling you to weaken or finish off multiple enemies at once. Since this scales off AD it is a good choice early, as AD is easy to scale quickly. The transformation occurs at 100 bonus AD, at which point the number of missiles increases from 6 to 12. this should be achievable quickly with the following build path:
THIS PORTION IS UNDER MAINTENANCE, FURTHER PLAYTESTING IS REQUIRED
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