Do not be put off by Devourer on Rengar. Late game, it will deal more damage than Warrior and you will have more sustained damage in fights afterwards. However, your mid game bursting will take a hit. If you buy it early enough when nobody has resistance, you could potentially do a lot of surprising magic damage. This item is also good if you have a magic damage light comp. Coupling this item with Blade of the Ruined King is an extremely strong late game build.
One of the biggest things you need to learn how to do is use Hydra whilst you're in mid-jump, but not before you've jumped. If you use the Tiamat active while you are jumping, you will deal damage at the point you are jumping to and not where you currently are. Using the active on top of your empowered Q will effectively chunk half a carry's health. Following that up with a normal Q will usually ensure the kill.
If you need a tanky engager instead of an assassin, Rengar is surprisingly sturdy and can still clear the jungle easily with a 9/21 setup. He still deals a monstrous amount of damage, even with double or triple tank items. However, you still need to build assassin items for the first twenty minutes of the game. Otherwise, your ganks will most likely not have enough damage to ensure a kill. Whenever I'm going for a mix of the two, I still end up with 400 AD, 100+ of both resistances, and 3k health. In this setup, it is important to hit your W on as many champions as possible for the potential +250% resistances from his W. That's a total of 105 in both resistances at max rank.
Other than this, I'd suggest watching Ryan Choi. He's good to learn from.