DARK SOULS: REMASTERED

Dark Sun Gwyndolin is the NPC for the Blade of the Darkmoon covenant and an optional boss that can be encountered in Anor Londo.

In order to access this battle, you first need the Darkmoon Seance ring from the Catacombs. It's in a sarcophagus guarded by a large skeleton in an area behind a fairly obvious breakable brick wall.

Then, you'll need to head to the Darkmoon Tomb, which can be found at the bottom of the giant rotating staircase in Anor Londo. With the staircase lowered to the bottom, you will find a seemingly dead end room with a bonfire and a statue of Gwyn. Equip the Darkmoon Seance ring and the statue will disappear, revealing a path that leads to a fog wall.

You'll notice a carpeted area in front of the fog wall- approach it and Gwyndolin will talk to you. Choose to "kneel" on the carpet and you will be given the option to join the covenant, unlocking the associated achievement. You can then go through the fog wall to begin the battle with Gwyndolin.

This battle takes place in a straight hallway where Gwyndolin appears in the distance and uses three different attacks on you. Your goal is to avoid the attacks and make your way towards Gwyndolin, ideally dealing damage before (s)he teleports away.

The three attacks are:

1) Lob of multiple slow moving, somewhat seeking magical projectiles. Get behind a pillar against either wall to avoid them.
2) Charged up straight magical burst that you can roll sideways to avoid.
3) Volley of arrow shots that can be easily avoided by running forward at an angle.

Push towards Gwyndolin, avoid the attacks, and smash him/her when you can until (s)he is dead and the achievement will unlock.

As always, refer to http://darksouls.wikidot.com for specific information relating to items or locations.