Who am I?
I'm a gold 4 rengar player, I fell in love with this champion s10,I now have over 1000 games playing rengar in top lane and jungle. I love the satisfying feeling of tanking everything and healing it all back and one shotting anyone really late game.
Pros/cons
This champion is not the best, there's a lot of other champions that do more and do it better. Rengar is fun though and can one shot and carry games. Rengar has so much utility a root a cleanse a lot of damage can even go ap, the bad things about rengar is he is mechanically hard to learn, takes a lot of practice to get his combos down, he loses a lot of matchups, his ganking is so limited.
Team Composition
Rengar can go into almost every comp, you can build tank bruiser, ap, one shot AD. You should avoid playing with all squishies because you have to be the tank and engage depending on their comp they will delete you but you can tank for a little bit be sure to go stasis rengar for situtations like this. Samira has very good synergy with rengar I believe she can use her passive with his root and he can tank for her and deal damage enough to get everyone half or below for samira to clean up. while rengar engages samira is in the back auoting to get her ultimate. yummi is also very good on rengar she creates an escape with the healing and movement speed she gives you her r allows you to walk up and full combo them without a bush. Ivern also has great synergy with rengar because of the bushes he can put in the middle of the lane, the shield and knockups he gives. Lulu also synergies with rengar well because if you dive backline and you're about to die she can save you and allow you to get your double w back up to heal to full hp. Avoid Xin zhao , hecarim , Kayn, darius, diana, graves, trundle. all of these junglers will beat you in everything you do. except for assassin kayn you one shot him as well unless he goes a brusier item.
Runes
Run conq a majority of the time you can run elect for 3 squishies or more, Elect for one shot ap rengar as well. the runes are shown above. Take exhaust into matchups that do too much damage, it'll help you a lot especially if you struggle. Take ignite into champs that heal too much, like viego or aatrox, darius, yi can be beat after 6 with exhaust or ignite.
Start.
Buy a control ward and a red smite you do not need pots as rengars sustain is good. place the control ward at krugs in bush, this allows you to jump over from red to krugs and do them quickly. double w and smite to be full hp and to instantly kill the big krug. Rush essence reaver first every game but get serrated dirk first, the lethality makes you do a lot of damage early. You want to buy essence reaver every game no matter what that should be your first item. You can buy serpants fang if the enemy comp has a steraks or immortal shieldbow adcs, that you need to one shot.

Abilities
Rengar Q- is all of your damage, it's an enhanced auto attack that can be countered by teemo blind, double q is all of the damage in your combo, if you land the combo correctly you can triple q to one shot anyone.
W- W is a heal and the empowered W is a cleanse this can be used to clear camps fast and save your health late game you take no damage from baron from lvl 14+ if you W correctly, it can also be used to tank all cc and damage with an engage cleanse and run out for your team.
E - Rengar E is a 25% slow and his empowered E is a root, this is a good tool to gank early with, to allow your laner to do lots of damage or even kill. this also breaks scuttle shield instantly. typically though late game don't use this your other empowered abilities are much more useful.
R - With ultimate hunter your R has around a 45 second cooldown late game, 14+ You want to use R for pressure or to pick someone off out of position it's okay to use it just for vision.
Passive - Rengar gets teeth for killing each person on the enemy team, the AD spike is kind of insane, also if rengar kills khazix he gains AD as well.

Rengar can play top, or jungle. I prefer to play him jungle if I get autofilled top then that's okay too. Play aggressive , play for dragons, rengar level 3 is stronger than a lot of champs because you have 4 abilities instead of 3 with an empowered one. the path I use commonly is blue gromp red into an invade it works well because you're level 3 and they should be arriving as you get there or already doing it, so it's easy to kill a majority of champions, if the champ is a hard matchup for you don't try it, full clear, 6 camp to a scuttle give if that champ is trying to contest, it's not worth dying for. Pay attention to tracking the enemy jungler that's all that matters you need to know if he's invading or where they are at all times. If you don't know take your best guess and invade the opposite side, ex. xin is topside I will go bot take his chickens and then finish clearing my blue side. ping off top to sit under tower. laners with CC are good for rengar because you can root off of it making for an easy kill.

Try to catch waves that your laner misses as rengar whether he's roaming or dying. You need to be ahead you're the win condition. split push if your team is feeding and apply pressure, it gives them time to farm and attempt to catch up and you also get more gold from splitting and clearing waves to build more items and hopefully reach that one shotting point. in team fights look at their comp is there a carry? who is fed? jump on that person if you have the items to kill them in one full combo even if you die, it could win the teamfight, If you need to tank for your team make sure you use your w's and try to fight around bushes use empowered W over Q if you're going to try to survive more than do dmg. E and run away if possible. You can baron with one fed person on your team if you choose to around 20 minutes you should have lvl 14 at least and have essence reaver duskblade or eclipse, but W and use your empowered W on baron to not take any damage. avoid where it's spitting and the circles on the floor. Pay attention to if anyone is splitting on the map, if they are answer them or help your team with a 5v4 try to drag the enemy into river or your jungle or theirs there's lots of bushes around there.

Thank you for reading, Sorry I didn't put as much detail I will put more soon as I'm still testing builds like immortal shieldbow and ap rengar.