There are a number of ways to best Gwyn. Given that I am not adept at parrying, I chose a tank approach. Here, I am level 78, wearing the Paladin Armor Set +5, the Grass Crest Shield, the Ring of Steel Protection, the Flame Stoneplate Ring, and wielding a Fire Great Scythe +10. I also took advantage of the extra protection provided by the Iron Flesh pyromancy.
After defeating Gwyn, you are given the chance to acquire one of two achievements depending on the ending you choose. If you light the bonfire, you receive the ‘To Link the Fire” achievement. If you do not light the bonfire, and instead leave the are, you receive the “Dark Lord” achievement. I chose the former.
Make no mistake, this is a tough battle. Concentrate all of your effort on one boss because, once the first falls, the second will become enhanced and replenish all of its health. Ranged attacks and attacks that include fire seem particularly effective. I attempted this battle at least 15 times before finally achieving victory. I could not have done it without the aid of SlappySlap86. He was instrumental in distracting, and later defeating Smough. Thanks man!