For this achievement, you need to reach Anor Londo. To do so, you need to travel through Sen's Fortress, which opens after ringing both bells of awakening. The entrance is located across the walkway from the bonfire in the old church, where Andre the blacksmith is residing.
When you reach the top of Sen's Fortress, you will find the white fog door to the Iron Golem. Do not walk through yet, but first do TWO things. As the other guides suggested, go up the stairs in the same room as the fog door and kill the firebomb giant. He does not respawn. Second, and this is missed by the other guides, but makes the fight a lot easier, go across the walkway to another tower. Be sure to be in human form. In this tower, you can find the summon sign of Black Iron Tarkus. Summon him and then go through the fog door.
Black Iron Tarkus can easily tank the hits of the Iron Golem. He can in fact solo this fight for you.
During the fight, if you hit the Iron Golem in his legs for enough damage in a short time, he will stagger. If you do enough damage during this stagger, you can make him fall. He will sit on the ground quite long, leaving him open for all attacks, go at him with all you have. If you make him fall near the edge, you can actually make him fall off the fortress to his death. Activate the ring on the middle of the platform and watch the cutscene. The achievement should pop after that, when you have your feet on the ground.
**Note**: There is a bonfire when you drop from the wall at the second scorched place (The second place where the firebomb giant throws his bombs). You can get human here if you have humanity.
My first guide, so please leave a comment if you like or dislike it! Constructive feedback is always welcome.
Praise the sun!