i've had a lot of success with renekton as a mage killer, in teams that run AP top, jungle and mid, you can be a monster by getting merc threads, maw of malmortius and spirit visage as your first items, and only really needing sunfire to not be too squishy to the only AD threat (that with maw and sunfire you can beat up depending on which AD it is)
I tend to do decent in pretty much any situation, as long as gnar is not on the enemy team because he is a HARD counter to renekton
http://matchhistory.na.leagueoflegends.com/en/#match-details/NA1/1770375343/36772325?tab=overview
http://matchhistory.na.leagueoflegends.com/en/#match-details/NA1/1774465171/36772325?tab=overview
also since his fury changes, with maw of malmortius and AD runes, you heal for almost 120 per champion hit with your empowered Q, which is massive in clumped up teamfights (and against low sustained damage such as not having an ADC clobbering you down with autos all fight long)