User blog comment:TheTobarMethod/Why is Mid one-sided?/@comment-178.235.216.166-20130317161321

Getting better is the number one option. Counterpicks to these champions also exist and have already been mentioned, but as all counterpicks, they should not be taken too seriously (Kayle (for high early kill potential and ultimate countering their burst), Lee Sin(for the extremely high early kill potential), Wukong/Zed (do more damage in early trades than the champions you mentioned and equal burst late) to a lesser extent Jayce, Khazix, Talon etc.).

However, it seems like the answer you are looking for is "what to do when an assassin on an enemy team gets fed* - the answer to that is very simple - group!!! - assassins are normally rather weak in teamfights, so you should move as 5 and force objectives rather than letting the enemy assassin do what he does best - kill people out of position/on errands.