Counterplay to a fed khazix?

If he was focusing a squishy the would generally die before they could cast any spells. Most of the time i had about enough time to barrier before it was over. Wasn't exactly sure what to do when you can't actually react to the damage in time. Had a similar game yesterday against a semi-fed one as Tristana. I absolutely had to ult the second he got to me (rocket jumping was not enough AT ALL) stealthed or not and had to hope for someone to re - cc him if he attempted to pursue. Kiting was not an option without someone to lock him down. I understand how to fight qa fed rengar though most of the time you're still going to take an asston of damage as long as he gets to you. I don't think he was unhealthy or anything I'm just used to actually being able to do something against alot of damage. (If kat jumps on you knock her back and either gtfo or kill her before she can get to you again. ) Pretty lost here. I guess pink wards would have helped but that isnt going to be an option next patch so lets throw those out the window.