Not a real guide please dont vote

Notes:

LOL attack speed caps at 2.5 per second

Empowered Savagery allows him to gain 150% bonus AS for nearly 3 seconds.
High base attack speed requires exactly 270% bonus AS to hit the hard cap. His kit can grant him up to 198% bonus AS, which in such case leaves only 72% bonus AS to go via masteries, runes and itemization.

Build and use bonus attack speed on Rengar wisely - any attack speed that brings him over the 2.5AS cap is wasted.

AS
Ghost blade == +50%
Masteries == +4%
BL Cleaver == +30%

So dont get AS boots or phantom

Get black cleaver early and to kill squishys, the lower the armour the better,
Always get BC over BT UNLESS you need the lifesteal for sustain

At around 200 ARM and espcially late game go last wisper

Replace Bonetooth late with more AD, e.g BloodThurster

Late Game Damage:
Go bone tooth as AD so cheap and better the later in the game you are, even with no stacks, brutiliser is very slightly better than flat ad but only just and don't really need yoshis ghost blade if you have Randans omen, so BC for damage or BT for sustain