Assassins tend to have 3 things.
Assassins tend to be vunerable to
Khazix has burst and building armor can help but only if you plan to kill him. Running from him can be a abit difficult. Build enough that you can fight if you think you can kill him.
He evades damage by becoming invisible or hopping away. Things that let you see invisible champs can be very helpful against him. When he hops away Chase him if you can but if itll take too long don't bother chasing him through the jungle very far, even if you get the kill if it takes too long it wont be worth it.
He's got a good gapcloser. Keep this in mind all the time. Dont get in range if you aren't prepared, not even for cs, its not worth feeding an assassin to get 1 or 2 cs