This is my build for renekton i personally do amazingly good with him because lots of people don't notice that your rage makes your abilities stronger. I start off by getting ruthless predator because of the stun this is ur main damage ability. Its your strongest ability and in the gank you stun them then activate exhaust then u have your first blood. Renekton make sure you farm because your rage meter increases by empowering your abilties. If your low on hp when in the lane you could use your q to heal but it has to have 50 rage. use this combo that I use w then e then q then e again you should get your kill.

Use pretty much the same combo now before fights and ganks make sure u have ur rage meter half or full inorder to do damage. Now you would be using your ulti in order to have extra survivalability. Just dont think your invinsable though u can be killed ive done it sometimes and died. Your build should be about to be completed. you win by team work not by thinking your god so stay with your team and feed off the squishies and kill there guy that do the most of the damage.