Sometimes,with Rengar or any champion really you are going to have to gank. Rengar has a passive used for ganks and other exciting moments,Simple, hide in a bush and jump out once they are in your circle.You will then leap to them throwing Your easy (E),Therefor slowing them. Then you will easily pull off a (Q) then finishing them off with your (W). you will of course use your Ult (R) if they flash away, preparing up for a easy kill and a upgrade to your Tiger-tooth Item. Mostly if your are jungling you will only farm minnions if your "laner" is absent or currently recalling.