I am creating this in-depth guide to help benefit the lower elo players to better understand the assassination play potential that Rengar possesses, and when to make those plays. This is also my first guide, so please comment and CONSTRUCTIVELY criticize so I can improve this guide and any others I make.

Rengar is an extremely strong fighter from mid to late game, whereas his early game pressure isn't very strong unless you catch someone off-guard near a collection of bushes. Every kill or assist you get adds a stack to your Bonetooth Necklace making you stronger, and making it easier and easier to get more and more kills.

Pros + Very strong assassin. + Gets stronger as the game goes on. + Can remove just about any champ from the Summoner's Rift. + Infinite jungle sustain with Empowered Battle Roar. + Skill cap is very high and can be competitive in high elo with practice.

Cons - Nearly useless pre-6. - Needs stacks of Ferocity to make a strong play. - If hit with hard CC, you have no escape without Thrill of the Hunt. - Enemies receive an indicator that you are about to kill them.

I like to take the Greater Quintessence of Armor Penetration and Greater Mark of Armor Penetration, as the game progresses people will generally have some kind of armor whether it be from runes(scaling or flat) or items. I find Greater Seal of Armor work well for jungling, and Greater Glyph of Scaling Magic Resist for the extra protection mid/late game in case you are against an AP that can poke, such as Xerath.

Scaling Magic Resist Glyphs vs. Flat Magic Resist Glyphs

As a jungler who is MOSTLY farming until level 6, you shouldn't be in contact with an AP champ for the main portion of your pre-6 game, and once you hit level 7, scaling becomes more effective than flat, therefor leaving only 1 level where flat runes are more effective.

Rengar Mastery Planner

Ferocity

I take Fury over Sorcery because [Rengar] needs the extra attack speed for early jungle clears, and it combos well with Youmuu's Ghostblade and Empowered Savagery. I find that Double-Edged Sword works extremely well on Rengar because for the most part you are going to want to assassinate a carry (delete a scrub) before they can have a chance to retaliate making the downside of this mastery very trivial.

Cunning

Most people would take Wanderer over Savagery, but I like taking Savagery for faster clears early early game because Rengar is already a very strong mid/late game champion, but as I said before, not so much early on. So the faster you can get to that level 6, the better. Assassin is the best option for Rengar because the most important thing to do as Rengar is take a carry (delete a scrub) out of a team-fight before one breaks out. Runic Affinity can be taken, but with the jungle path I will be providing below it isn't necessary. Meditation has no effect on Rengar, don't take it... ever. Merciless is too good to pass up anyway; since you will be assassinating (deleting scrubs) they should be below 40% health, or else they won't be dead. Being a jungler, you probably shouldn't take Bandit , unless you plan to be covering a lane frequently, in a standard match you should take Dangerous Game for somewhat obvious reasons. As Rengar cooldown reduction is very important, however getting the +1% and max cap op 41% is unnecessary as you should be about 30% making Intelligence not very effective, whereas Precision will lower your TTK (Time To Kill) although only slightly. Finally your keystone mastery should be Thunderlord's Decree because being a combo assassin, you will gain the extra burst when going all in (deleting scrubs).

As Rengar you should use your Empowered Battle Roar as frequent as possible to keep your health points up. Enchantment: Warrior is a strong power spike for Rengar with its +60 Attack Damage, making the Greater Quintessence of Armor Penetration and Greater Mark of Armor Penetration more worthwhile. The jungle route provided numerically in the above picture is the optimal jungle path that provides the fastest level 6, most gold per minute, AND most effective use of the buffs themselves. This may sound like a ridiculous claim, but it has been mathematically proven to do all of the above. This route is the most commonly used in Diamond+ ranked, however it leaves your buffs open to be stolen by champs that invade such as Udyr and Shyvana so make sure to ward up.

General Jungle Tips

When clearing a camp kill the smaller minions first as they deal more damage per hit point that have compared to their larger counterparts. (This tip does NOT apply to your first/initial leashed camp).
As Rengar try to take advantage of your Unseen Predator passive to quickly leap to a jungle monster to cut down on your jungle time.

How to Execute this Route Appropriately

Blue Team

Start at Ancient Krugcamp smiting immediately, then rotate towards Crimson Raptor camp, after clearing Crimson Raptor camp get your Rift Scuttler. Ward your Red Brambleback and get going to your Greater Murk Wolf, afterwards grab your Gromp with Smite and optimally grab your top side Rift Scuttler. If you are positive that the top side Rift Scuttler is already gone then immediately go back to base to save some more time. If you see a blatantly obvious kill opportunity do not be afraid to take it, just be careful as you are not very strong until level 6. (You should be able to grab your Stalker's Blade and go back to jungling).
On your second clear start at the bottom and work your way up again smiting ONLY Ancient Krug camp and Gromp however on this run do NOT get Rift Scuttler's and make sure to pick up your Red Brambleback and Blue Sentinel.

Red Team

Start at Grompcamp smiting immediately, then rotate towards Greater Murk Wolf camp, after clearing Greater Murk Wolf camp get your Rift Scuttler. Ward your Red Brambleback and get going to your Crimson Raptor, afterwards grab your Ancient Krug with Smite and optimally grab your top side Rift Scuttler. If you are positive that the top side Rift Scuttler is already gone then immediately go back to base to save some more time. If you see a blatantly obvious kill opportunity do not be afraid to take it, just be careful as you are not very strong until level 6. (You should be able to grab your Stalker's Blade and go back to jungling).
On your second clear start at the bottom and work your way up again smiting ONLY Gromp camp and Ancient Krug however on this run do NOT get Rift Scuttler's and make sure to pick up your Blue Sentinel and Red Brambleback.

Little Known Fact

Once Rengar achieves his 6th stack on Bonetooth Necklace his leap range is increased to the point that he can leap to Dragon, Rift Herald, or Baron from over their respective walls from brush with sight as shown in this video
Play Video (this video was originally posted by 'ArmouredTeddy' on YouTube).

Split-Second Combo

Rengar's '1-shot' combo is a little complicated without practice, but easy to learn.
Start with 5 stacks of Ferocity and use Thrill of the Hunt when going in to assassinate (delete a scrub). NOTE: Make sure to check your map for any other nearby enemies Thrill of the Hunt will allow you to see nearby enemies even if they are stealthed! When you get in leap range of your target you should activate Youmuu's Ghostblade and your Empowered Savagery and the instant you begin your leap cast Bola Strike upon landing cast Battle Roar and immediately activate Tiamat or Ravenous Hydra. If your target is still standing, smash your Savagery like your 'Q' key is a spider. It should look like this
Play Video (this video is posted by 'JKSAD on Youtube' on YouTube). NOTE: The video is old, but the combo IS the SAME.

Team-Oriented Combo

Again start with 5 stacks of Ferocity and basically do the same combo, but instead of using Empowered Savagery you would use Empowered Bola Strike. NOTE: This combo is use for initiating a fight on a SINGLE target that you CANNOT fight alone. This combo is good for taking out champs such as Dr. Mundo, Garen, and Volibear.

Which Combo Do I Use?

If you are assassinating a single target (deleting a scrub) that you can handle alone use the Split-Second Combo.
If you need your teammates to take on a single target use the Team-Oriented Combo to stop the target and allow your team to help deal out the damage.

Special Tip

This is a little tip I use when in a tight situation, and don't really see in any other guides. Let's say you want to kill a low hp Vayne with enough to barely survive a quick combo, but she could auto attack you once to kill you. She is near a bush and you want the kill; you should use your Unseen Predator leap with your Bola Strike, Savagery, Battle Roar, and IMMEDIATELY Flash back into your bush so you can get an extra auto attack yourself with a second Unseen Predator leap, possibly even another Savagery or Empowered Savagery. NOTE: This combo is very situational and has a high risk, moderate reward, although it has saved me in a few fights, and merited me quite a few kills.

Rengar's kit revolves around single target damage, but when teamfighting use the Split-Second Combo to take out the AD Carry or the AP Carry, try to take out whoever deals the most damage to the team overall, don't prioritize a champ that does high damage to you and no-one else because your team has to fight whoever is remaining, your job is to get out after killing 1 target unless you have additional EASY kills available. If you finish off an easy kill in the back-line then have a tank/front-line coming after you, use the bonus stacks you get from Thrill of the Hunt ending and use your Empowered Bola Strike to keep him in the back of the team and run. This renders the tank/front-liner useless and takes a carry out of the fight.

Special Synergy

Orianna is an amazing champion to pick alongside Rengar. With Orianna's Command: Protect on Rengar, he can leap into the enemy team with Thrill of the Hunt to give Orianna the perfect opportunity to use Command: Shockwave essentially winning a teamfight immediately (deleting the whole scrub team).

Nami is a very nice pick for a Rengar comp as well. With Nami's Tidecaller's Blessing on Rengar deals further increased damage on his Savagery/ Empowered Savagery which can be immediately followed up with a nice Ebb and Flow for damage and heals as well as Aqua Prison and Tidal Wave to make an all in seem easy.

Major Crowd Control

Alistar, Leona, and Malphite are all good pick as they can hard CC the enemy long enough to allow you to do whatever you need and to get out.

Divers

Akali, Diana, Wukong, and Xin Zhao are all amazing diving champs that can go all in with Rengar so you won't take all of the damage.

Q1: I do so well in game, but my team loses, why?

A1: Even when Rengar gets fed, he has an extremely hard time solo-carrying. If your ahead, try spreading the wealth by giving your lane carries kills with the Team-Oriented Combo, rather than taking it for yourself.

Q2: I build different items, why should I build yours?

A2: The item build I have provided is just the items I have personally experimented with and have come to find work very well for ME. If you are a player who doesn't remember to use actives Youmuu's Ghostblade probably isn't a good option for you (although essential to Rengar, just using as an example).

Q3: Will this guide be kept up to date?

A3: I plan to come back and update this guide at EVERY patch and whenever I think something needs to be added/edited/updated.

Q4: This guide is missing some key things that make some guides better than others, why?

A4: This guide is still a work in progress, I am releasing it as is for the time being because some people just want a build and why, there is definitely a lot I still plan to add, don't worry.