Hi, I'm RegniX and this is my first guide :) I'm Bronze I at the moment, but don't judge me, my carry percentage of Kha'Zix is 75%.

Pros:

It has everything you need, invisibility, slow, jump, skillshot.
The Q has short cooldown.
The double jump helps you to dive.
Evolve.

Cons:

Long cooldowns except Q.
Squishy (1 stun enough in tf -> Mercurial Scimitar).
Hard to master.

I use the basic Armor, Magic Resist, Damage runes, and 2 Quint of Damage, and 1 of Lifesteal, you can pick 2 lifesteal, if you want to.

I use this masteries for Kha'Zix:

I mostly start with W (the exception is invade -> Q), the second is Q, you can help to kill the smaller golems with Q. You can spam Q, it has constant 25 mana cost, with blue buff, you won't see the cost of the W too, but be careful, de W cost very much mana! The third skill is E, after that, max out the W first, then the Q, finally the E. The evolve order is E->Q->W.

After you started with the proper items, you should start at blue. You need mana for the W, and the cooldown reduction is also good for him, it can save health potions -> better sustain. Don't use the smite immediately when the Golem spawned, wait the 390 health. Get the wolves, after that kill the Elder Lizard, you will hit lvl 3, so you have all of your skills what's needed for a gank. Gank a lane, mid if possible. If you go for another gank, go through the jungle, and get some camps before ganking if possible.

Before the teamfight poke them with W. In the teamfights, use your ultimate, to regain your passive, your next attack slows and damages more. Use your E carefully, it has long cooldown. Use the Hydra to push, and stay in the range of the explosion of the W, because it heals you.

I was RegniX, if you have question, ask me. Good luck on the Field of Justice! :)