I am tired of this champion, If I dare go into detail as to why I dislike playing against him, I will be called bad and dismissed instantly, so I will make an equally pointless statement. Seriously fuck 
and everyone who plays him. In the most basic of terms, he is unhealthy as fuck, and not fun to play against in any situation as any role.
Fed Rengar's are pretty scary. Being able to one shot squishes out of no where and leaving little time to react before he does so is pretty hard to counter. But he's kinda like Riven in the sense that if he can't snowball he usually can't do much. It's hard to come back with him. So take the time to invade on him, get a kill while he's trying to take wolves or raptors and shut him down before he gets started. Or play someone like Shaco that pull of a level 2 solo invade when he's weak from clearing a couple camps. Might work out better than you think.
Early game rengar sucks and he has no escpae, So try to invade his jungle even if you are a laner.
