This is a trophy for completing Episode 4: Unger (which consists of only one Order, No. 39).
It tasks you with escaping a WWI battlefield with trenches. You also will meet and face Clifford Unger, the man from the flashbacks after resting in private rooms.
It is recommended to bring blood bags, assault rifles and grenades for the encounter at the terminal in South Knot City prior to the cutscene where the chiral tornado takes you to this time frame. Throughout the trenches after an easy run through a battlefield you will see various shotguns, grenades and assault rifles to pick up. It's impossible to run out of weapons during the fight. Use your Odradek to sense where Unger and his skeletal soldiers are in the maze like area. It will emit an orange glow when you are near him. Aim for the head and don't stop shooting until he collapses to the ground on his knees. The skeletal soldiers do not need to be killed to have access to Unger to take damage but don't let them creep up on you and pile up, especially if you're playing on Hard.
It will take a few encounters in the trenches (4 I believe) until the battle is over. It is not terribly too hard but may require a few attempts if you're unfamiliar with cover based third-person shooting. Most people should have no problem.
Good luck. Sorry I don't have a video.