Upon completion of The Gilded Cage, you’ll see a golden “?” icon in Saint Denis so head over there to meet some guests from the party. Arthur agress to assist Rain Falls in obtaining a file from Cornwall Kerosene and Tar up near Valentine. You’ll have to wait a few in game hourse before you can resume the mission so it’s a good idea to take stagecoach to Valentine, it’s a long ride to Eagle Flies and you’ll also pass the in game time in the process.
Following the cutscene after meeting up with Eagle Flies, quickly dash down the hill and jump into the back of the wagon. Wait until the wagon reaches it’s destination and then climb out, crouch walking to the door in front of you. Keep crouched the whole time and look at the radar to see the direction the guards are looking, the first guard walks to the left and the second follows him shortly after. Pass through once they’re gone and take the stairs up to the second floor. There’s just one guard to avoid here, wait for him to turn his back and then crouch walk behind him to reach Danbury’s office.
Interrogate him by beating and then threatening him to get the File that you’re here for and then escape through the window. As Arthur lands, he finds himself in a spot of bother but thankfully, Eagle Flies’ diversion provides the opportunity to use Dead Eye and dispatch the three guards. You’ll now need to make your way over to Eagle Flies so begin dashing, only stopping to shoot the guards in the immediate vicinity and with perfect accuracy to maintain above 80% for the Gold Medal.
You’ll find Eagle Flies on horseback here with your own horse close by, quickly mount up and follow him away from the factory. You’ll be pursued by some mounted guards, turn around to face them and use Dead Eye to kill all of them before continuing the escape.