Act 1 - "The Bog":
At the start of the level, stick to the right and head through the gate. Continue forward to the building ahead. Cpt. Price will order you and Roycewicz to take the building. Pvt. Roycewicz should be to your right at the base of the stairs. Let him go up the first flight alone, and he will be attacked by an enemy. Simply shoot the enemy without killing Roycewicz or letting him die.
Feel free to ask any questions, and add me on TT/ TA/ PSN/ XBL as Danokitty. Happy hunting!