Difficulty: Easyish
This is story related and unmissable. However, it can be tricky due to the Executioner boss at the end.
You can either avoid the boss until the end, or keep downing him with explosives and gunfire and punching him when he's stunned.
Note: There's lots of ammo around and exploding barrels to make this easier + a machine gun near the end point of the larger area (It's on the right in a case).