The cheat "airwalking" has already been explained on a previous cheat, but you can do more than just walk on air with this cheat.
On the desert map, follow all the intructions for airwalking as mentioned already in a previous cheat. Once you can walk on air, get a jetpack, and rise to the top of the map. You can land on anything that is as high as the map. Then, just walk off the edge of the map, toward your base.
Once you can see your base underneath you, look straight off of the map and you should see some extra scenery mountains just floating in mid-air. Gently bump against one of these to make you descend slowly. Immediately turn back toward the map while descending. You will eventually drop to under the map, where you can see the actual physical location of your enemy's spawn room. Make sure you are above it before you stop moving, as you can not walk through walls.
In the enemy spawn room, your enemy is unable to harm you. You can get as many kills as you want in your enemies spawn room, and you can even pick up their ammo in the spawn room. Because they cannot hurt you, the only way you can die is by suicide (launching a grenade too close to yourself, etc.)
Caution: If you descend to far or too fast before you reach the spawn room, and miss it entirely, you will just keep falling and falling forever. If this happens (which sometimes it does) you have to exit the game.
This one is hard to figure out, but worth it. It's also pretty neat to see the map from the outside, and walk off the map.
[more cheats for Perfect Dark Zero here]