Please note some achievements are buggy.
When leaving the room Tabula was you should meet Langham. Interact with the revolver and with Langham and then interact with him again to start a discussion. Reveal the truth to him and he should leave. Head out to the balcony area and use the drain pipes to go down. On the drain at the bottom interact with the radio and you should climb back up. Use the hammer on the guard when he is close enough and knock him out. Head back to the chapel area and exit through to the cable cars.
Video guide if you prefer: