Army Of Two

All primary weapons
Successfully complete the game on the Recruit or Contractor difficulty to unlock all primary weapons (assault rifles, shotguns, etc.).

All secondary and special weapons
Successfully complete the game on the Professional difficulty to unlock all secondary and special weapons.

Professional mode
Successfully complete the game on the Recruit or Contractor difficulty to unlock the Professional difficulty.

Switch characters anytime in Campaign mode
At the campaign menu, select "Start New Campaign". Select the character you want to play as, then go back to the Campaign menu, and select "Continue Campaign" to start with the character you selected. -From: Erik Zalewski

Easy money in China
Start the China mission, and successfully complete the first objective for $30,000. After you have completed the objective, go to the checkpoint, which is back down the hill next to the hovercraft by the dock. Once you get to the checkpoint (which saves your money), quit and restart the mission. Repeat this as many times as desired.

Intel locations
The following is a list of all 14 briefcase locations. You get a cash reward for picking up each briefcase. They can only be collected once, and disappear in future playthroughs once they are collected. You can check "Objective" in the pause menu to see how many briefcases you have collected for the level. Collection is accumulative; you do not have to collect all briefcase in the first playthrough.

Afghanistan

1. Behind the first M-11 missile is a small uphill path; follow it all the way to the end to find the briefcase.

2. After encountering the first heavy armor guard in the mission (a cutscene plays showing him walking out of a door), kill him and all the guards, and search the platform area. The briefcase is up past a short set of stairs at the back of the platform.

3. In the area just outside of Brian Hick"s cell, go all the way to the back to find the briefcase.

Iraq

1. Right before ascending the ladder to take out Ali Youseff"s Lieutenant"s Helicopter, look around at the bottom of the ladder to find the briefcase.

2. When you are asked to call in the air-strike on the bunker, run to the tent nearest to the huge wall dividing the area (there are some sentry towers near the wall). The briefcase is on the side of the tent facing the wall.

3. In the final area of the mission before meeting Ali Youseff, there are some metal platforms with guards on it. Kill them all, climb up the small steps to the second level of the platform, and then the ladder to the third level to find the briefcase.

Aircraft Carrier

1. On the main deck of the ship after you land, run behind the control tower and to the right to find the briefcase on a platform on the side of the ship.

2. In one of the hangars, you have to operate one of the planes to blow through a door (a heavy armored guard appears once the door is blown). Search around the wall closest to the door. The briefcase is behind some missiles.

3. In the final hangar, after activating the missiles to sink the ship, look around the back right of the hangar to find a basketball court. The briefcase is behind the basketball hoop closest to the hangar wall.

China

1. Once you arrive at the dam, go up the stairs towards the first control panel. Look across the dam to see a similar structure directly opposite. The briefcase is there.

2. Once you arrive at the village, co-op snipe the two guards, and take the right path. At the first house you can enter on the path, head to the second floor to get the briefcase.

3. After the only back-to-back sequence in China, do not enter the factory. Search outside near the back of the factory (it is behind a dump) to find the briefcase.

Miami

1. After the gates to the luggage area are opened, you have to run to the second floor to operate a control panel. There is an office right before the control panel. Search around the back of the office to find the briefcase.

2. On the third floor of the SSC tower where you have to jump onto the lift, ignore the lift, and keep running forward to the back of the floor to find the briefcase. It is directly opposite and across to the staircase that you came up on.

Defeating strong enemies and Bosses
An easy way to kill heavily armored guards and Bosses is to use one of the rocket launchers, preferably the FIM92 Stinger since it can lock-on. It should only take one hit to kill the guards and Bosses. -From: Erik Zalewski

Metal Gear Solid reference
The MSG-90 sniper rifle has a barrel type named "MGS Sniper-Wulf", which is a reference to the Sniper Wolf Boss in Metal Gear Solid.