This is just a simple build SUGGESTION that has worked for me so far. First I go for wriggles for cheap lifesteal. Then I go for brutalizer and phage. I complete the phage and then complete the brutalizer into the youmous or start a maw. In the end I either get a last whisper or a Guardians angel.
When I evolve his abilities I prefer to go Q,E,R. If anyone likes the build and wants me to expand on it leave it in the comments. I'll see if I can expand it at a later date.
I chose the items that I did because I find that kha'zix is able to deal out significant damage if he isolates a foe even without building full damage. Due to that I decided that building him with survivablilty was the way to go. The frozen mallet provides him with health and the CC he lacks. The Wriggles provides him with Armor and Cheap Life steal. The Youmous and LAst Whisper give him excellent armor pen and some good stats. The Maw provides him with magic resist and damage. The Guardians Angel keeps you alive and gives an average amount of resistances.