Hi, my name is Holyshiftlock.
I've created a few guides but not much response.
Rengar is an awesome champion. Both in jungling and top laning.

You want to have a good amount of Armor Penetration, this way u can max the damage done.
Ofcourse Attack Damage is you're main stat. But as u see, Rengar has a low amount of health. So he's easy to kill.

Pros:
Rengar's Ferocity stacks 5. U can do heavy damage with Savagery, do less damage but heal urself.
Or u can root someone in place with Bola Strike. I prefer using the Bola Strike. This way u can encounter ur enemy or finish him off without him running away.
Cons:
As u may have seen, Rengar does not have a lot of health. He can easily get killed by AD carry's
Counters Of Rengar:
-Teemo
-Ashe
-Caitlyn
-Kog'Maw
-Varus

Last hit minions. Thats the tip i have for you guys.
This way u can max the amount of gold received. The more gold you have, the more damage u do.

Jungling is easy with Rengar. If you're gonna jungle with Rengar, don't take Blue Buff in the first place. U dont need this buff yet. Get you're blue buff for a teammate. Or in enemy jungle if possible. Rengar is good at Jungling, but u need to make sure u have enough lifesteal before encountering Red Buff's , Blue Buff's or Dragon solo. U need to have atleast ur Bloodthirster and Black Cleaver, this way u have enough Attack Speed and enough Lifesteal.

Smite; Only for jungler's.
Flash: To make sure u can escape, or finish off an enemy. Flash to him and use ur Bola Strike, this will slow the target.
Ignite: Use this if ur enemy is running away. If he has low health u might get the kill with Ignite.

Teamfights are hard to do with Rengar, as he can easily be killed. He will be foccused in teamfights. He can do much damage. But has low health.

Use Bola Strike to slow/root the enemy in place. Then go to it and use Battle Roar and Savagery. If he's running away and u dont have any spells to use. Use Thrill Of The Hunt to chase him down and kill him. Don't turret dive with this champion untill level 14 atleast.

Try to keep healthpots with you at any time. They can only come out usefull. When playing against a counter, stay at turret and wait for jungler to gank before engaging. Try to outfarm youre opponent. As you do, harass him with Bola Strike.