Hi, my name is Alex and I am a regular League player, today i'll bring you a Kha'Zix guide.
''Kha'zix is a melee assasssin who utilises quick movements and invisibility to take down his foes.''-David Turley (Riot Phreak)- from Kha'Zix Champion Spotlight
Kha'Zix is a really unique champion, fact that he can evolve his skills trough the game is a very nice thing that makes him very powerfull.
If you like to take down foes very fast; have a huge mobility ; enter & exit fights without big scratches Kha'Zix is a must for you.
Here you have the champion spotlight:

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And the Ciderhelm version of it:

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Pretty standard, if you want more sustain ( playing against high damage opponent ) you can max W first, but you need to be close to minions/monsters/champs to heal yourself.
If you want early plays, at level 6 you can evolve E, a huge gap closer evolved + you have a very good reset which can save you or bring you another kill.The reset comes when you get a kill or a assist (only evolved version of E => very important)
In any match make sure you evolve Q, it increases the damage on Q and range on both basic and Q.
Make sure you put a point in your ultimate at every chance (level 6;11;16), this will allow you to upgrade your skills.
Normally, you only are allowed to upgrade 3, but if you kill Rengar at level 18 you get the 4th upgrade.

Pros

+ Huge burst + Great roaming + Good sustain + Great gap closer/escape

Kha is a great pick in solo queue, he is a assassin with high kill potential and great mobility.He's got amazing escapes with , high juking potential with , nice sustain & range poke with and the most important huge damage with .

Cons

+ Hard laning vs ranged + VERY squishy early + Position realiant + Only 1 skin :(

Like every champion he's got cons. He's got hard laning versus ranged champion and he's very squishy early game.

-Try to CS as much as you can: It's very important as an assassin to have much gold, because, of course you are item reliant.
-Go for kill when target is isolated: you deal more damage with Q. Of course, if your opponent is low and you'r sure you can kill him, he doesn't need to be isolated.
-Use leap when target flashed/used escape, because if you first leap to him, it will force you to flash to get the kill, but another time, if the target is low first leap then do your combo to get the kill.
-In team fights don't engage, wait for other teammates to do that, because you will get bursted very fast.
-In team fights look forward to kill ADC/APC if they are not protected by other teammates, this will make the teamfights easier due to de ADC/APC huge damage.

Ironically, Kha'Zix works well with Rengar.
Other strong picks with Kha : Xin Zhao ; Morgana ; Blitzcrank ; Kassadin
Kha'Zix is strong against : Kassadin ; Twisted Fate ; Fizz ; Teemo ; Yasuo
Kha'Zix is weak against : Lee Sin ; Rengar ; Jax ; Jayce ; Yorick ; Pantheon

So, this is the guide, i apologize for grammar erorrs and poor design, but you know, this is my first guide so i'll get better in the future.
Hope you liked my guide and it brought you victories on soloQ or normal matches.