While Ninja Raiden is in sneaking mode, walk up to a unconscious guard and enter first person view. Send a thrust (stab) into the enemy's head. This will wake him up. Kill the guard and hide his body so the other soldiers will not find it.

While Raiden is in sneaking mode, walk up to a unconscious guard and enter first person view. Aim at the enemy's head, then keep punching him. This will wake him up. Kill the guard or he will call for back up. Hide his body so the other soldiers will not find it.