Dark Sun Gwyndolin is a completely optional boss that is extremely easy to miss. There are two ways to go about being able to fight this boss. His boss arena/covenant is located at the bottom of Anor Londo. Push the elevator lever one more time to get there. There is a statue there but either method will make the statue invisible so you can proceed. The first method is killing Princess Gwynevere in the Chamber of the Princess. The second is by getting the Darkmoon Seance Ring from the Catacombs. I recommend the former, however. To get the ring, go across the bridge that was covered with spikes and through the door. Sprint through to the corpse in the grave for the ring. The statue in Anor Londo will disappear if you have this ring in your inventory. Entering the fog will also prevent you from tanking up in the Darkmoon covenant.
The fight itself has you chasing him down a seemingly endless hallway. He will cast spells at you and they are easy to dodge. The blue ball will devastate you but it will devastate you even if you are blocking. Simply move out of the way and it won't hit you. He'll also shoot a barrage of homing soul arrows. Hide behind the pillars so they don't hit you. When you get close, he'll shoot a burst of crossbow bolts at you. Block these arrows so they don't devastate you. Attack when you get close and when he teleports away. Repeat until defeated and the achievement will unlock.