Renekton is ridiculous 0_0 I said "Sure I'll give him a try" without looking at a guide and for the first 10 minutes I got owned, then I managed to break evens by the end (we still lost). In the next 2 games I was just a teamfight monster, and winning every 1 on 1. If he's fed I reckon just keep him at arm's length and harass as much as possible before entering a fight. That stun is unbelievable, especially is you have a Trinity Force and CD reduction.
Still haven't looked at a build but I was running with Ionian boots, Ghostblade, Trinity Force, Warmogs, Inf Edge, Atmas. I reckon the Warmogs is necessary at some stage.