Wtf is with Kass on ARAM?

I was playing TF with the build the pros use and was doing good considering they had Kass, Raka, Veigar, Xerath, and Lucian but EVERY SINGLE time Kass would jump on me I would exhaust and he would still do 3k+ damage to me and I would get about half his hp and then we finally lost after it being close. He would DIVE 1v5 and get away with it like wtf? Why haven't they given the "ARAM nerfs" to him yet? Is Exhaust just that useless to where I can stun someone twice, have the lifesteal elixir, exhaust, full build, and STILL not be able to kill him or is the champ just broken