Not Updated For Current Season
Start with an Ancient Coin, I know thats not the meta item, but trust me, if your laner is farming well, you will be several items ahead of the opposing support by mid game IPROMISE THIS. Try to buy the Ninja Tabi first since they really cancel out damage really well early game. From then get a Frozen Heart. After you build that, scope out how the other team is doing, if the opposing spellcaster is fed or does insane damage, you might want to buy some magic resist (Banshees Veil, Locket of the Iron Solari). If the opposing spellcaster isnt doing so well or alternatively, they have no spellcaster, then youre gonna go full armor. After the Frozen Heart get a Sunfire Cape, and after that a Frozen Mallet (you can also get a Rylai's, but I personally prefer the mallet). Once you get those 2, you can go either Warmogs, or Randuins Omen depending on the situation. If they have a fed Trynd. for instance, a Randuins might be the way to go.
Play defensively at first. Once you get your Ninja Tabi, you can be more aggressive. Try to be the initiator of engagements, especially if the opposing support cant save their adc. Use E to engage or wait till the right moment and Q the adc (Pop W before engaging) then as the adc is fleeing, use your E to stop them in their tracks. Use R at your own discretion.