Story related, cannot be missed. After you ring the two Bells of Awakening, the gates to Sen's Fortress will open up. Go to the bonfire one story above Andre of Astora's workshop to get here. Once you get to the top of the area, carefully drop down off the small gap in the fence up the stairs at the top for a bonfire. Get through that area to face the Iron Golem boss battle.
The Iron Golem is a simple fight in and of itself but it can be made even easier by doing these two things:
Summoning Iron Tarkus. His summon sign can be found in the room across the small walkway to the right of the boss fog.
Killing the Giant who throws firebombs at you during the fight. From the boss fog, turn around and go up the staircase to get to the giant. Kill him for a titanite chunk and he won't throw firebombs at you during the fight. He won't respawn.
The fight starts with Iron Golem blasting a wind vortex at you. You can dodge it or run behind the piece of debris in the center. Get underneath him and hug the backs of his left leg. Just focus on doing as much damage to him as possible do you can stagger him for an easy kill. Iron Tarkus can help greatly here as his attacks do large chunks of his health and stagger the Iron Golem. Simply keep attacking and moving out of the way of his very slow attacks and you'll win!
After the fight, interact with the golden ring in the center of the boss arena to be taken to Anor Londo for the achievement.