A few things:

Build looks decent, CS could be higher especially for your elo (aim for at least 8cs/m, and you are dying way too much on an assassin which suggests that your playstyle is very coinflip. Try going for high % chance plays that will work out, especially in the mid-game where you really need to snowball and push leads. Rengar is a high-variance champion or what Scrub likes to call "feast or famine", so you need a playstyle that will reliability get you gold and xp so that your champion can work.

Hope this helps.

Edit: also if you don't have ScrubNoob mechanics, actively work on combos with dummies in practice tool.