All of the information I am getting for this guide is from Scrubnoob and I am not him. Scrubnoob is a challenger rengar player. You can find his stream on youtube here: https://www.youtube.com/channel/UChchKMJe-ROGuFvKwvu9Fvw and the new account he plays on here: http://na.op.gg/summoner/userName=Elo%20Terrorist other than that please upvote this guide and post in the discussion section if you need any help or clarification.

In 8.20 Scrubnoob has been running fleet footwork for extra damage and healthy clears. Triumph for the survivability after you get a kill. Alacrity for the extra attack speed and coup de grace for extra one-shot potential. For the inspiration tree he runs magical footwear for the free boots and +10 movement speed and cosmic insight for extra base cdr to ability spam. The reason you run fleet instead of dark harvest or electrocute is because it is more helpful early and the extra attack speed is really nice. Also you should be dealing enough damage with fleet footwork so the extra electrocute damage isn't very helpful.

Scrubnoob likes to start machete and then rush tiamat and then finish the jungle item. Tiamat is really good for early clears since alot of his kit single target. The boots you pick are situational and I see Scrubnoob running basically every relevant pair of boots there is. You then want to run duskblade, youmuus and choose whichever last item you want after upgrading tiamat to ravenous hydra.

You want to start Q W E and max Q W E but Scrubnoob likes to max E second for no particular reason.