Champions that are slower mid champs suck as Swain, Ryze, or Brand are easily countered by Kha'zix who can dodge their main damage abilities and continue to do damage. The w on Kha'zix allows you to farm without having to trade with you opponent. this also lets you whittle sown your opponent until you can go in for the kill, remember that Kha'zix does more damage to isolated targets, so keep the minions away. Kha'zix does extreme amounts of damage and can get away with it, making him the ideal assassin. you also will only need the first or second blue from your jungle before being able to go on by yourself.