Hi, welcome to my short rengar guide.
I made this build after being frustrated at watching people build him differently and failing hard.
You do not have to follow this build exactly bud this is a great way to get high kills and if played right minimal deaths, thanks for reading and enjoy and ask any questions if you need anything.

On every wall of his den, the trophy hunter Rengar mounts the heads, horns, claws, and fangs of the most lethal creatures in Valoran. Though his collection is extensive, he remains unsatisfied, tirelessly seeking greater game. He takes time with every kill, studying his prey, learning, and preparing himself for the next encounter with the one monster he never managed to defeat.

Rengar never knew his real parents, but was raised by a human who was revered as a legendary hunter. He was an ideal pupil, intently absorbing the lessons of his father, and improving them with his uncanny feral instincts. Before his mane had fully grown, Rengar set off on his own and claimed a wide territory for himself. Along its perimeter, he mounted the skulls of his slain prey - a warning to would-be aggressors. He thought undisputed reign of a region would fulfill him, but instead, he grew restless. No beasts in his domain proved challenging prey, and without formidable adversaries to push his limits, Rengar's spirit waned. He feared that no worthwhile game remained, that he would never again feel the thrill of the hunt. Just when things seemed their bleakest, he encountered the monster. It was a disturbing, alien thing, distinctly out of place in his world. It bore huge scything claws and devoured any animal that strayed across its path. Overzealous at the prospect of a challenge, Rengar ambushed the monster in haste. It far outclassed anything he'd hunted before. Their fight was savage, and each suffered crippling wounds. Rengar lost an eye, but the most grievous blow was to his pride. He had never before failed to make the kill. Worse yet, the severity of his injuries forced him to retreat. Over the following days, he hovered on the threshold between life and death. He was wracked with pain, but beneath it, he felt a glimmer of joy. The hunt was on. If such powerful beings existed in the world, he would find them, and stack their heads high. The monster, however, was a kill he wanted to savor. On his den's largest wall, he reserves a space for the beast's head, a trophy he swears will one day be the centerpiece of his collection.

"Prey on the weak and you will survive, prey on the strong and you will live." -Rengar

Pros

+Unbelievable burst damage.
+Naturally kinda tanky.
+Unseen predator is a great way to gank and re enter fights when both you and your opponent are low health.
+Very short cooldowns
+Slow (Stun) is an awesome way to get you that boost the get your kill.
+Great lane sustainability thanks to his ferocity battle roar which heals.
+If played right you frustrate your enemies to the point of shouting "OP!".
+Amazing at farming.

Cons

-His ultimate can be easily countered by an oricles or pink ward (if still in lane)
-Other high damage dealers can get the best of you if you don't enter with the correct burst.

Unseen Predator (Passive) - While in brush or stealth, Rengar will leap at the target when using his basic attack. Rengar builds 1 Ferocity with each ability he uses on enemies. When reaching 5 Ferocity, Rengar's next ability becomes empowered, granting it a bonus effect.
Range: 600 or 750 with 9 trophies on Bonetooth Necklace

Unseen Predator has no cooldown and the leap can be used repetitively. For example, when chasing through brush, Rengar can continually leap onto his target. The buff will also last for ~.5 second after leaving brush or stealth. Upon reaching 5 Ferocity all of Rengar's empowered abilities become available for use. Note that his cooldowns are not reset. After Rengar consumes his Ferocity, all of his abilities resume what remains of their cooldowns.



Savagery - Rengar's next basic attack deals bonus damage and grants him increased attack speed for 3 seconds.
Cost: None
Cooldown: 6 seconds
Physical Damage: 30 / 55 / 80 / 105 / 130 (+1.0 per attack damage)
Attack Speed Bonus: 30 / 35 / 40 / 45 / 50 %


Empowered Savagery - Savagery deals bonus damage equal to 150% of Rengar's attack damage, and the Attack Speed bonus is doubled.
Cost: 5 Ferocity
Cooldown: None
Physical Damage: 30 / 55 / 80 / 105 / 130 (+2.5 per attack damage)
Attack Speed Bonus: 60 / 70 / 80 / 90 / 100 %

Savagery is your main damage ability and Empowered Savagery ends up doing a ton of burst damage and a lot of sustained damage through the attack speed boost. This ability will also reset Rengar's attack timer so landing an auto-attack before using it will maximize the damage. Also, the attack speed buff from both Savagery and Empowered Savagery stack, so using them back to back is encouraged.



Battle Roar - Rengar lets out a battle roar, damaging enemies and gaining bonus armor and magic resist for 3 seconds.
Cost: None
Cooldown: 15 / 14 / 13 / 12 / 11 seconds
Magic Damage: 75 / 120 / 165 / 210 / 255 (+1.0 per ability power)
Bonus Armor & Magic Resist: 20 / 30 / 40 / 50 / 60


Empowered Battle Roar - Battle Roar also heals Rengar for 15% of his maximum health.
Cost: 5 Ferocity
Cooldown: None
Magic Damage: 75 / 120 / 165 / 210 / 255 (+1.0 per ability power)
Bonus Armor & Magic Resist: 20 / 30 / 40 / 50 / 60

Battle Roar helps you stay tanky in fights without having to build a lot of defensive items. Rengar gets his sustain for Empowered Battle Roar which will heal him even if the ability does not hit an enemy. These two versions of the ability will not cause the armor and magic resist to stack, but will refresh the duration.



Bola Strike - Rengar throws a bola at his target, slowing them for 2.5 seconds. The slow decays over the duration.
Cost: None
Cooldown: 12 / 11 / 10 / 9 / 8 seconds
Physical Damage: 60 / 105 / 150 / 195 / 240 (+0.7 per bonus attack damage)
Slow: 50 / 55 / 60 / 65 / 70 %


Empowered Bola Strike - Bola Strike initially roots his target for 1 second. The slow is applied in full after the 1 second, and decays over the remaining 1.5 seconds. Crowd control reduction will reduce each duration separately.
Cost: 5 Ferocity
Cooldown: None
Physical Damage: 60 / 105 / 150 / 195 / 240 (+0.7 per bonus attack damage)
Slow: 50 / 55 / 60 / 65 / 70 %

Bola Strike is Rengar's only CC ability. This is a target ability and applies full on-hit effects such as the ranged slow from Frozen Mallet. This ability can be priceless during ganks, especially if Empowered Bola Strike is used to snare the enemy.



Thrill of the Hunt (Ultimate) - Rengar activates his predatory instincts, stealthing and revealing all enemy Champions in a large radius around him. He gains movement speed and generates 1 Ferocity per 0.75s while stealthed. Lasts 7 seconds or until he uses an ability.
Cost: None
Vision Radius: 2000 / 3000 / 4000
Cooldown: 120 / 85 / 50 seconds
Movement Speed Bonus: 10 / 15 / 20 %

Thrill of the Hunt is what makes Rengar so amazing, it's great for ganking, surprise attacks, escaping, just everything. I prefer to use this when in a sticky situation and wait till i get the furosity boost and attack again and destroy in one quick attack!

Just pick whatever you feel is best for you, keep in mind you have a built in heal and flash (ulti) so you can just go nuts really. :P

Reminder, this is just how I play and find effective, you may be different.

Early game

When you start with a dorans blade you always have that little extra burst over the other champ you're against (assuming they go with boots and pots) so you can harass easy peazy from the bush with your passive. Remember your furocity bar and harass everytime you get it full or if you're on the downside just heal, they won't be able to sustain aslong as you without going back. When you first go back remember to pick up wards, even though you have your ult for a get away its always nice to just give that extra **** you to their jungler.

mid game

By mid game you should have a good portion of your items built and that mean DAMAGE TIME! Do not ever engage the team fights cause you are not a tank, no matter what other builds say you are not that tanky! Try stay at the back, in the brush if near and just jump on their carry, assuming you get some kills and ahead you should be able to take them out in a surprise attack in a few hits and then just steam roll threw their whole team, easy as that.

Late game

By late game (if it gets that far) you should have no problem back dooring cause of your ults low cool down, one on one'ing cause you are RENGAR THE BEAST and surviving in team fights. In my experience when I'm playing awesome and my team don't be fuds, the match doesn't go past the thirty five minute mark.

Like I've been saying I've had amazing results with rengar and as soon as I figure out how to upload the picture I will. My best result by far to date is an 24-1-14 in a 35:32 minute game I've also had 21-0-8 in a 32:37 minute game, 15-0-4 in a 21 minute game and like I said if you play well and keep your ult for the right moment you'll survive without issue. Sorry about lack of proof, my first guide lol.

This is my favourite champion in league, he is just so fun to play unfortunatly you get idiots you make him look average when he really in near on the brink of superman.

Please leave comments, feed back, anything really just to make this guide better.

Hope to see yous out there rocking this build, cheers.