First i evolve Taste their fear to get great damage against low health enemys and against isolated enemys. (You can upgrade Void spike if you need to farm a lot)

Next i evolve Leap to get as much mobility as i can. It helps you to get in and out of the battles. And if you kill or if you assist a kill your Leap's cooldown is refreshed. So basicly you can jump into battle and kill an enemy and just jump to safe again.

Then i upgrade my Void spike or Ultimate.
If you choose Void spike it will help you farm, and apply Unseen threat.

If you choose to upgrade your Ultimate(I prefer this), you can now activate your ulti 3 times within 10 seconds, and take 40% reduced damage.

+ Huge damage output (If you know how to play with kha)

+ Great mobility

- Kha'Zix is very vulnerable.

- Great for killing enemy carrys, but vulnerable against other bruisers.

What seperates a good Kha'zix from bad Kha'zix is that good kha'zix player knows how to use Passive right.

Unseen Threat

When Kha'Zix is not visible to the enemy team, he gains Unseen Threat. Unseen Threat causes his next basic attack against an enemy champion or Evolved Void Spike to deal 15 / 20 / 25 / 35 / 45 / 55 / 65 / 75 / 85 / 95 / 110 / 125 / 140 / 150 / 160 / 170 / 180 / 190 (+ 50% AP) bonus magic damage and slow by 25% for 2 seconds.

Unseen Threat lasts until consumed and has no internal cooldown.

For example if you sit on a bush and jump to an enemy you will make bonus damage.