rengar support electrocute really !

i think u can counter him with shaco bard pyke he so week against theme especially shaco + using exhaust on Rengar when he comes out of his ult will reduce his damage by 50% preventing him from 1 shotting your carrys and avoid dueling a Rengar when his resource bar is near full, as his skills will become empowered.
It looks ridiculous, but it's a nice boost. It gives ADC effortless score because it has dealt a lot of damage.