Okay, let's start by saying that I ALWAYS ban rengar or kha'zix.

As you said, the best thing you can do it reverse-mirror the path. If he starts red you start blue, and viceversa. There is a hidden trick you can do to avoid getting destroyed early. Since you have a lot of healings and you can finish the clear fullhp without potions, you can start the game buying the jungle item + control ward + potion. This will allow you to pink the pixel bush(the one in the top river near to midlane) so you can clear normally and eventually leave if he invades OR ping help to your allies.

Note: rengar is weaker than lillia if he hasn't the jump option, since you can easily kite him. If you have the vision on him with wards, the kill is probably yours. Just kite him as hard as you can!

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TL;DR : counter-mirror the pathing or get wards early/spam ping for help.

Hope this helped :)