well Rengar works really well with Cooldown Reduction and Critical Strike as it helps him use his Q more often and more damage (Yes Q can Crit atleast the AA Part)
so lets dive into the Guide!!!
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Hey i am Dharcness and this is my guide to CRIT Rengar
why you ask? because this way you can delete people easier than with your standart build

Runes:
9x Mark Of Armor Pen - Its kinda standard on assassins
9x Seal Of Armor - Its Standard In The Jungle
6x Glyph of CD - It gives you 5% Cooldown at Lv1
3x Glyph of Sc. MR - MR is not really needed in jungle
3x Quint Of ATK DMG - Pretty Standart

Masteries:
Ferocity: 6
Fury - Gives Attack speed for early clears
Double Edged Sword - deal more dmg

Cunning: 18
Savagery: kinda meh but it can help
Runic Affinity: cuz secret stash gives you mana but you have no mana...
Merciless: for 2shot or something
Dangerous Game: to continue fightning
Intellegiance: +5% BONUS CD AND MAX CD OF 45% INSTEAD OF 40%
Storm Raiders Surge: to chase if they are not blown up

Resolve: 6
Recovery: regen when below 20% is really nice to stay in jungle
Tough Skin: block 2 dmg from minions and monsters, kinda meh but helps you survive

+Gives your opponents heart attacks
+Incredible Burst
+Multiple Choices of Verocity all are usefull
+The real meaning to "Death Brush"
+Great Ganks around bushes & Past Lv6

-Squishy
-No Escape
-if falls behind he is kinda bad

you can start either side and get gromp/krugs + buff then you can go get river scruftler and then if you are healthy enough you can go get the other buff or go bank and rush the Hunters Potion, from their on just jungle and gank

Mostly going in solo and kill a target who is caught out of place (mid/adc/jgl in a far pushed lane) and just assassinate them