To get infinite ammo for the dual-handguns, equip the dual-handguns with at least one bullet when you have all weapons slots full. Locate a weapon that is not in your inventory on the ground(not on top of anything). Hold SQUARE to trade the dual-handguns for the weapon, then switch back one handgun and grab the other. The number of bullets you previously had for the dual-handguns will double.
If the an enemy does not know you are there, holster your weapon and melee the guards. With some luck, the fallen guard will make little noise, and they will not look at you when you do it. You can dispose of the most of the guards without firing a shot.
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 bunch 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 him 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 multiple T.P.S. reports.
To avoid shellshock, always look around in a paranormal vision first before moving on. Also, if Alma is in her child form, go towards her to move on. If she is in her older form, run away. It is also a good idea to shoot while running.
Pistols are great for defeating nightmares. They can take them out in one shot, compared to other weapons that require multiple shots (except heavy weapons). This is also very useful due to the fact that certain times, when Alma and Fettel take you to hell, you will lose all weapons and get a pistol.
Make a mark of some sort to lead you back on some Medkits that you didn't have room for before. That way, you can find them again if you run out. 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.
Use your Medkits at the correct time; don't 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 absolutely necessary. Just run to cover while using your slow-mo abilities. Your health will regenerate slowly back to 40 if it drops below that, however you must be out of the firefight to regenerate your health.
Since the assassins are hard to see due to their camouflage and high speed running, try using slow-motion with L2. You will instantly notice them once they are slowed. After that, let all of your rounds into them.
If you do not have slow-motion (for example, in the bonus mission when playing as the demolition man), fast fingers and a good eye are required 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 brief moment. 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 approaches directly in front of you. When he reveals himself, start firing at him.
Wait for slow motion to get up to full and have a shotgun. When ready, activate slow motion and fire as many rounds into the armor as you can. During this, it is also a good idea to circle around it to throw off its fire. This tactic is not useful if slow motion is off. The Particle weapon, Repeating cannon, and Rocket Launcher also does significant damage against them. If done correctly, Slide Kicks will kill a Heavy Armor in one hit.
Use the semi-auto rifle or plasma rifle. Zoom in and shoot it in the head, then in the upper legs where there is no armor. The Heavy Armor soldier will double up each time and cease shooting. Repeat your shots from head to upper legs until it is dead.
Holster your weapon when they appear and use slow motion. Then, use a round house, slide, or bicycle kick when they are close.
Use the following trick in Interval 8, Mission 2 during a Stealth Soldier attack. When you enter the warehouse, kill the first one that is in front of you and let the second one to hit you. Watch it go away, but keep an eye on it. It will come back and jump across the ravine in order to reach you. When it does jump, use slow motion and a round house kick.
Corner yourself so the Stealth Soldiers cannot sneak up behind you. A wall will work, but a corner is better so you have a better view.
A shotgun will works great against any Stealth Soldier.
The weapons you find will always indicate what you will fight up ahead:
WHAT IS AHEAD
This does not means anything, as they are found throughout the game.
Indicates that a platoon of enemy soldiers is close by.
This indicates that long distance firing will be needed soon, or heavy armor is close by.
Indicates possible snipers or heavy armor.
One pistol indicates Paranormal activity is nearby. Two pistols do not mean anything.
Indicates that lots of enemy soldiers, mechs, etc. are near. Basically, there is a lot of everything coming up. It also indicates that the end of the game is near when more quantities of them are found.
Indicates that a Mech, Leviathan, or heavy armor is close by (not so much Heavy armor).
This does not usually mean anything, as they are found throughout the game. However, it may indicate heavy armor.
This indicates that there will be lots of any type of soldiers nearby.
When you find some ammunition, drop the corresponding weapon. Collect the ammunition, then pick up the weapon again. You will gain more ammunition than if you had only collected the ammunition by itself.
In the last interval, be careful when Alma goes after you in the last vision. On Hard mode and above the rooms will be pitch black, allowing Alma to hide and attack you without being seen. Pay close attention to anything that appears to be moving in this last vision. Also, the flashlight and slow motion are useless there.
In the harbor mission, you will find a bunch of consoles with "Power and shutdown." There are, however, one or two machines that have wording similar to "Coins collected, secrets found, lives spent, ammo used, etc.". It is a gamer's console.
After the firefight in the gas pipes area close to 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 and 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 to notice that it is the same one from an office level from Shogo: Mobile Armor Division.
Before the first apparition of Charles' face, check the broken down door right over the table and look into it to hear Alma crying. However, you usually cannot see her.To actually see Alma, go back to the pair of double doors you saw before you saw the broken table. Open and close them repeatedly and eventually the "Incoming: Unknown Origin" will appear. Keep doing this. After a couple more minutes, the doors will disappear and you will enter a paranormal vision.
After defeating the group of men and a cyborg wielding a Penetrator in a large room, go through the doors and onto a balcony with plants underneath. Use the ASP rifle to kill the two men that are attacking from below. Do not jump down to where they were as there is an very large cyborg with a multi-rocket launcher waiting for you. Instead, jump directly across from the doors you just came through and hide behind the first obstacle you find. Get out from your spot and shoot the machine with your ASP once to draw its attention so that it comes towards you. Once you get its attention, immediately get back to your hiding spot. When you hear it has stopped walking, lean out from your spot. It will not notice you, even after you start attacking. Keep shooting it until it explodes.
To skip the scene with Fettel kidnapping Alice (even though it is an apparition), just shoot Alice or Fettel. You will, however, find an additional scene with Alice's body on the floor (also an apparition). This might be a sign of the future though because Alice's body is discovered in Interval 10 in the same way.