Its very simple one just farm and try to gank some lanes even if you die dont worry if your team survive 25 mins you are a monster.
Rengar is glass cannon he oneshoot but he is oneshooted too with this build you still can oneshoot some squishy enemies and dont die.
Pros / Cons
Pros : gr8 mobility , tanky , good damage , strong late
Cons : you need to know rengar mechanics , hard to play , not good early