Here is a tip, instead of play a squishy adc with zero purpose besides glass cannon, play a hard engage support, like 
how does this support counteract rengar? its quite simple really, A blitz sees a exclamation mark appear over his ADC's head, he must first hover his "Skill Shot" grab at the adc. Now this may seem weird to some, "Why would i aim my skill shots at an ally" you may ask. Well, the only person whose important is the adc carry, so it makes since a rengar is gonna jump on them right? So when rengar jumps on your valuable adc, you simply grab the kitty by the tail, and release the full combo of electricity into said kitty, killing the cat and saving your adc with one combo!!!!
Next time, on why rengar isn't broken, "Bubbles with Nami"