This can be done in any difficulty. The first level (Courier) begins with 2 objectives: Collect equipment from the safety deposit box, and avoid civilian casualties. From the begining of the level, head toward the safety box, but before opening the box to get your stuff, punch, or stun (but do NOT kill) the security guard standing near the box. You will see a "mission failed" message on your screen, and Robinson will tell you to come back because the mission was compromised. Still, take the box, and walk away from the level. When you leave, you will still have completed the level. The reason for this is that if you harm, but do not kill the guard, new objectives stops appearing, so even though you "failed" the mission, the two initial objective (Take the box and do not kill civilian) are still sucessful, and thus, the game "thinks" that you suceeded in the mission.