This guide is more focused on the actual boss battle, there may be some slight spoilers here, especially if you want to be surprised by the bosses.
Right before you get this acheivement, you must fight two huge gargoyles. While it is possible to take on by yourself, the easiest way to defeat these monsters is to summon the phantom of the knight, Solaire of Astora. Solaire will then be able to distract one of the beasts while you take care of it's twin.
To summon, you must have met Solaire in the Undead Burg and received the White Summon Soapstone. Once you get to the Undead Church, use a humanity to reverse hollowing at a bonfire, and then climb to the top of the church. Right before crossing through the white mist, there will be a marker on the floor to summon Solaire.
Other than that, this is a fairly straight forward fight. They seem to be weak against lightning and have a fire breath attack, so come prepared.