You can't rework Rengar to not oneshot. That's the point of assassins, burst down the enemy squishy fast. It's the same as saying all assassins are op because if they get fed they can oneshot. OF COURSE they can oneshot if they are ahead. Stop crybabying. You can never be pleased, can you?
"Some if the inherent frustration is intended- it's their job to kill you and dying isn't fun." -paraphrased from Scarizard, 6.12 patch notes
Triggered rengar main right here You realize getting bursted in under one second is not what Riot wants They want it to be 2-3 seconds.... You are the crybaby for having your champion having criticism
How to stop Rengar from one shotting you
If Rengar one-shots squishy, that's their problem. If Rengar gets 1K AD and one-shot full tanks, that's Riot problem.
Before you ask, yes, I saw Rengar with 1000 damage and yes, he really did one-shot a full tank while also killing another target with his now AoE Q. We have Jhin to get 1K AD, not Rengar, since Jhins kit is made around getting outrageous AD, while having restrictions to it. Rengar have none expect for being a glass cannon. And the excuse - "He is an assassin!" I mean sure, I take that, he can one-shot squishy target let's say ADc. It can be countered. But he is assassin not shreder to one-shot full tank.
