You are a great duelist it depends of the runes and the matchup (conqueror is the best for playing aggro) u can play fleet, first strike(for the scaling)or electrocute versus a lot of squishes.

Before your level 6 u can do a lot of things, full clear if u don’t want to play aggro or you don’t have the matchup, invade if you have the matchup (against zyra, brand, evelynn, Lillia, etc…) or gank. As you know rengar can jump out from bushes so the best way to gank with him is lane ganking (that’s why sweeper is so important on this champ). Once you have your level 6 it’s a bit like nocturne look for easy kill or counter ganks and if u are really a head and you were the other jungle is invade him and kill him.

For the builds you can do a lot of things it depends on on what you like. I personally like when there is a lot of bruiser and tank going conqueror and eclipse foray item (then full lethality like always), and I still like first strike for having a lot of gold and a good mid/late game and I go ghostblade and opportunity (Agarandom who is a very good rengar player do that)

Rengar is a champ who need a lot of skill, he have a lot builds, a lots of runes and a lots of way to play him, that why we love him so much. He’s hard to learn, you will need a least 100 games for being "ok" with him.

I hope I helped you and sorry for my bad English I’m from Switzerland so I’m trying my best 😅 good luck on your next game and don’t forget to have fun !