Just simply deff and farm, try to dodge spells and try to stay on full hp.
On 3 lvl you can try to trade, but remember, KHA'ZIX's PASSIVE IS VERY IMPORTANT.

Try to gank other lanes, on 6 lvl you are incredibly powerfull, but remember to do not be greedy for kills, use brain!
Remember to upgrade Q first, with R you can do incredible plays, remember to use youre passive!

If You are fed just try to take down enemies in jungle solo or even try to fight 1v2 if you are feeling like pro, but be carefull, even when you are feeded you can easy die if they will stun or snear You.
If You are not feeded so much try to fight 1v1 in jungle, that will make Youre team fights much much easier.

If You can, take down solo adc or mider, use simple combo like E W Q hit R hit Q (obv kill on paper) and next use E to escape, it will be succesful because of E upgrade, when you kill or assist in kill, youre e will be instantly ready to be used again.

My english is just bad, im not sure if you can understand all of that. I hope you will enjoy my very short guide, its my first one, so i can not say its the best Kha'Zix guide, but everything i said in this guide is my experience, i have already more than 220 games with this champion. I will try to update and upgrade this Guide. You can leave questions in comments. Bye! <3 :)