Most players wouldn't rush a Tiamat but I find this very helpful when farming. I get the Mercury Treads because the easiest way to kill kha'zix is usually to stun, slow, or snare him. The Maw Of Malmotius and Warmogs is so you dont die instantly and the other items don't need to be explained.

As the assassin your team will rage at you for cleaning up all the time but thats how kha'zix usually gets fed. But don't wait for the fight to end before you go in. Thats why you get the Tiamat and keep it late game. It helps in team fights. Often have someone making sure u don't die when you enter the fight.