Enemies are considered isolated if they have no allies within a short distance. The damage of Taste Their FearTaste Their Fear is massively increased against these targets.
This includes neutral monsters so it is recommended to kill off the lesser monsters first so the tougher, final monster will take considerable extra damage from Taste Their FearTaste Their Fear. However, this tactic should be used selectively as it does not noticeably reduce clear time, and you may even incur extra damage by letting the tougher monster live longer.
Unseen Threat activates when Kha'Zix cannot be seen by the enemy team, so you can reactivate it by using brush or Void AssaultVoid Assault. This can be used as an effective form of harass in lane or to provide steady but heavy damage output.
If you have invested an evolution point into Void AssaultVoid Assault, you can use the brief 40% damage reduction to reduce the impact of significant burst damage when timed correctly.
When ganking before level 6, try to keep Leap available in case the enemy uses Flash or other escape skills.
From levels 6 and up, use Void AssaultVoid Assault as soon as you leave the brush in order to surprise the enemy. Use the second cast (or the third, if evolved) for its mobility in case you need to chase or flee.
When you evolve abilities you are adapting for 4 different play styles:
Evolving Taste Their FearTaste Their Fear grants you massive burst damage against isolated champions on low health; useful for counter jungling, defending against opposing counter jungling or landing a quicker killing blow on an isolated champion.
Evolving Void SpikeVoid Spike grants you a very powerful harass and farming potential early game. The slow works both defensively and offensively to slow enemy champions. The explosive radius of Void Spike is effectively tripled, if shot correctly, allowing for maximum damage output. (Enemies can only be damaged by one explosion, the same goes for the heal on Kha'Zix)
Evolving Leap grants you immense ganking potential, due to the extreme range (roughly two times Flash) and the instant refresh on killing/assisting a champion. A useful tactic is to jump to an isolated enemy, slow them with Unseen Threat, do maximum damage with Taste Their FearTaste Their Fear and jump away upon kill, ready for a gank on another lane.
Evolving Void AssaultVoid Assault grants you an extra use of the Ultimate before going on cooldown; essentially granting you an additional use of the passive, the speed bonus and 1 more second of stealth. It will also grant you a nice damage reduction from all sources except true damage, including poison from Singed or Teemo and AoE burst damage. However, timing is critical, as the reduction only lasts for the 1 second periods of stealth.
Trinity Force also proc the damage buff every time you activate Void AssaultVoid Assault, if you wait the 2 seconds cooldown of the item between one activation and another, so if you evolve active camouflage when you use your ultimate you get a three burst of damage within 4-5seconds.
Tiamat is great on jungling with Kha'Zix, along with ganks. The mana regen helps him when he doesn't have blue, the extra attack damage scales with his abilities, and the 50% splash damage helps isolate larger jungle creeps in order to maximize damage. Also, the splash damage increases the chance of isolating a ganked target, increasing your damage against them.
If the two rows of spikes on his shoulders have gotten bigger then Kha'Zix has evolved Void SpikeVoid Spike, causing him to shoot three pairs of spikes instead of one and allowing them to apply his passive's slow and damage.
If a pair of wings are stretching out of his back, then Kha'Zix has evolved Leap, greatly increasing its range and instantly refreshing it if he gets a kill or assist.