Getting far into the game so snake has a large health bar really helps with this mission. Mostly what you need are movement and grenade cards, so make sure you have plenty of movement cards and at least some grenade cards. You need to get the guards to follow you so if you have a grenade in your hand when you start toss it somewhere near the guard who is where you start the stage in story mode. If not, just run up in front of him until he calls an alert. Make your way toward the bathroom slowly, making sure the guard follows you, but not quite close enough to shoot you. The other guard should see you when you get near the bathroom. Go in the bathroom and move past the sinks, stay in front of the far left urinal. Wait for both guards to enter, then face away from the urinal and just keep discarding until you get a grenade. Toss the grenade into the spot where it's blast radius covers the 3x3 area in front of the door and the guards should move into the right position.
Turn 1 : Equip M1891/30 in LEFT equipment slot. Equip M9 in RIGHT equipment slot. Turn 2 : Attach Sniper Wolf to M9. Attack quick draw to M9. End turn. Turn 3 : Move forward three blocks. Equip M9 on same slot as other M9 to load and shoot guard. Mission Complete.
Turn 1 : Move forward three blocks, place C4. End turn. Turn 2 : Move forward two blocks, place C4. End turn. Turn 3 : Move forward on block, left two blocks, face the direction You came from. Fire RPG7 at guard two blocks forward and one left from your position. Mission Complete.