I have level 5 mastery with Rengar and man is he good. Tell me if im wrong but is it good that i build 4/5 ferocity, then ult then go q, q then q again? I am hoping someone can help me get even better with him. What happens if there are 2 40 percent health champion? Who do i go for? Can someone help me be a better Rengar player pls?
Warning: The Gameplay Board absolutely hate Rengar and will downvote you for no reason other than you mentioned Rengar, but not next to the word "broken" or "nerf".
I don't play 
often, I play 
ADC, so take what I say with a grain of salt.
There are many combos to one-shot someone, but one of my favorite is to build up 5 ferocity ->ult ->use Q before leaping out of stealth -> jump on squishy -> use E and tiamat active in midair -> use W(instant) in mid-air -> autoattack lands, if they aren't dead then instantly Q and keep autoattacking: the attack speed from the empowered Q makes this happen quickly.
If there are two low health champions you generally want to go for the higher priority champion. If you have enough damage to kill one of them instantly before they can react, kill the one who is more fed and does more damage, because then hopefully you can duel the other champion with ease, or if your team is following up your team will have the advantage.
Finding relatively isolated carries and assassinating them is fine, if you can get away with it. If you jump into their team, take out their carry, but then their team kills you, maybe your team can win the 4v4. Maybe. It's only really good early-mid game where you need the gold to build up your one-shotting items, or if the carry is fed and the shutdown gold will be worth multiple enemies getting assist gold.
A good way to teamfight with Rengar is similar to all other assassins, except you have stealth. When you want to start a 5v5 teamfightHave your team move aggressively towards the enemy, then when the enemy carry walks up to join the fight, take him out. The tanky members of your team will close in either before or during this time, so they cannot focus you without being killed by your team(the fight is 5v4 and this point) and unless the enemy is way ahead of your team (for example if there are other, still-alive carries more fed than your entire team), you will win the resulting fight. If you're late to the fight on the other hand, it should still be okay as long as you can still kill them and they haven't killed any of your team members yet.
At 5 ferocity, ult>activate Q before leaping, E>W>Tiamat mid air, Q again once you land, this gives very little reaction time, auto attack them with the bonus attack speed for the 2 seconds until your ferocity fills back up, then empowered Q if needed.
Ok thx you guys! Real help
I am a bit late but I would caution you against ulting with 4 stacks as it makes you unable to snare mid leap. It is situational but i like to have that choice available to me
Managing to get through the ban phase is a pretty good one
