Enter "F3ARDAY1" as a profile name to unlock all levels. Note: Achievements are disabled when using this profile name.
This trick requires the dual handguns, all weapon slots full, and at least one bullet for the dual handguns. Have the dual handguns equipped, and find a weapon you do not have in your inventory. This weapon must be on the floor, not leaning or on top of something. Hold Square to switch out the dual handguns for the new weapon, then switch back for one handgun and grab the other. You will notice that you have more handgun ammo. For example, if you had one bullet for both and you used this trick, you should now have two bullets. If you repeat the trick, it will increase to 4 then 8, 16, 32, 64, 128, then the maximum of 200.
Try to make a mark of some sort to lead you back to some Medkits that you could not carry before. If you so happen to run low or out of Medkits because of a big fight you can find them again. This is useful in Interval 11 with the armies of ghosts coming after you. Just make sure that you are able to backtrack to get the Medkits you left behind. Additionally, use your Medkits at the correct time. Do not use Medkits immediately when you have about 70 to 80 health remaining. Try using a Medkit when your health drops below 30. One Medkit can heal up to about 30 to 45 points. If your health happens to drop below 40 and you are in a 1v1 or 2v1 fight, do not bother using a Medkit unless you really need to. Just run to cover while using your slow-motion abilities. Your health will regenerate slowly back to 40 if it drops below that; however, your health does not regenerate during a battle.
Because the assassins are difficult to see due to their camouflage and high speed running, try using slow-motion (L2). You will notice them immediately once they are slowed and can then start firing at them. If you do not have slow-motion (for example, in the bonus mission when playing as the demolition man), you must have a quick finger and a good eye to notice them running around. There are two ways to do this. The first is to toss a grenade to reveal the assassin for a certain amount of time. Hopefully the splash radius will either hit him or kill him. Note: The grenade has a two to four second delay before exploding, and correct timing is required. The second way is to wait until the assassin is directly in front of you. When he reveals himself, start firing at him.
While making your way through the large office building, look for papers hung on the wall or scattered on desks. Many of these papers have "TPS Report" written on them, as well as a symbol including three boxes at the top. This is a reference to the popular movie Office Space in which the characters are involved with TPS reports at their office, Initech, which has almost the same logo as the symbol at the top of the papers.
While making your way through the large office building, about midway through the level there should be a large room filled with cubicles, with multiple offices on the side. One of the cubicles in the middle of the room has a lot of boxes piled into it, with a red stapler on the desk. This is in reference to the character Milton in Office Space. Management is always making Milton move his desk and using his area for storage, as well as stealing his red Swingline stapler. This can be found in the same area as the multiple TPS reports.
After the firefight in the gas pipes area near the exit point of the map, there will be a gas pipe on fire. Turn the object off in the next room, then return to the now extinguished pipe area..There is a semi secret walkway there. Walk in, find the ladder, and climb up. You will find a health booster, and there is a door on your left. Go into that office. Listen to the song. It is the main theme from Shogo: Mobile Armor Division, an anime FPS created by Mololith. Look at the dry marker board. It is the same one from an office level in Shogo: Mobile Armor Division.