Just after you defeat the El Gigante, you will backtrack to the church where you will put the Round Insignia on the door to unlock it. Once inside the church climb the ladder on the left hand side, jump across the balcony using the chandelier and operate the colour wheel puzzle on the other side. Here you will want to match up each colour to the center insignia on the wall. This puzzle is very easy. Once the gate opens go through the door that the gates were blocking. Ashley will be in this room, and upon entry a cutscene will take place and you will be awarded the achievement!
Credit for this solution goes to an unknown user for the same achievement on the Xbox 360 version of the game.