Your final encounter with the Colossus will be in chapter 4 and once you defeat him, you will earn this achievement.
It will bounce around from column to column throwing chunks of church at you. You need to charge your glaive with fire from the burning rubble and then hit the Colossus with it to make it fall to the ground. At this point, you must run up to it and press whichever button it tells you to. Once you've done this 3 times, it will start charging around on the floor. Just keep shooting it and hitting it with your fire-charged glaive and it should die fairly quickly.