The best way to fram With rengar (top) is to jump to a minion from a bush, then use w.
because the enemy will not react that/when you jump out of the bush. So the enemy cant deal damage to you becasue his reaction will not be fast enough to focus you. Normaly you will just get outtraded by enemy because he`s basic attacks deals more damage. but when you jump from a bush he will not have good enough reactions to deal damage.
Rengar mid and late push is amazing. you can Clear a Whole wave With jus one w.

You dont deal a lot of damage to chmaps,you will probebly not get a lot of kills, but you are a great team player. You will get a lot of assists. Because you dont deal a good amount of damage, dosent means you cant 1v1 People. because of Your w you will get a lot of hp back after using Your w while active(5/5)