Army of Two Cheats

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.

An easy way to get this achievement is to shoot while you are dropping down by parachute. Purchase the RPG, and play the mission where you are trying to land on the Navy ship. At the bottom you will see three guards. Shoot at them as soon as you can and before they disperse. Once you kill them, restart from the last checkpoint. Repeat killing the enemies until you get the achievement.

Keep restarting the Afghanistan level. Four or five martyrs will show up during the back-to-back section. After that, if you restart the checkpoint, one will appear if you boost up onto the compartment and kill them. Make sure to aim for their chests and not their heads. You can only get the achievement by blowing up the bomb packs on their stomachs.

Get on one of the gun turrets you encounter, and keep firing. This will fill the Aggro meter and keep it full for the one minute requirement to get the achievement. This is easier to do during the second mission.

You can get this achievement with the M134. As soon as you have it, go to the Afghanistan level. After you pass the first wave of enemies, you must go through a tunnel. If you are playing alone, keep your AI behind where he cannot shoot enemies. Run pass the first set of sand bags, and get to the second set in front of you. Use the M134 to shoot any incoming enemies. Then, just keep shooting until you get the achievement. If you are playing in Co-op mode, do the same thing; one player stays behind and the other one shoots. Restart to the last checkpoint if the other player also needs the achievement.

Play the first level. While going through Aggro Training you will have to use your partner to distract an MMG turret that is being fired by Pedro. Shoot at the turret for a full minute to raise your Aggro. You will have infinite ammunition, so you do not have to worry about running out.

Killing Heavy Armor soldiers in Aggro Training counts towards the achievement. If you have already completed the game, go through the Somalia mission. Playing through Aggro Training about three times should be enough to get the achievement.

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... 

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

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

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

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.

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.

Go to the China mission, and have your AI hold position. Run up the hill to get full Aggro, and kill all the enemies. Shoot the bridge, then run back down the hill. Get full Aggro again, and kill all the enemies. Reach the check point, quit, and restart the mission. You should get the achievement in approximately fifteen runs of the mission.

Earn enough money to buy every mask variant. Then, buy every mask to get the "Man Of Many Masks" achievement; however, before leaving the buy masks screen, turn your Xbox 360 off. When you start the game again, you will still have the achievement and all your money back.

An easy way to get this achievement is to shoot while you are dropping down by parachute. After the first level when you are parachuting down, take out your sniper rifle, and start shooting. Once you kill all enemies and before you land, pause the game, and restart from the last checkpoint. Repeat killing the enemies until you get the achievement.

In the training level after the back-to-back lesson, you will be told to run around the corner. There one of you will bleed out. Do not have your partner heal you. Instead, sit there and shoot all the guards. You will not die and their bullets will be ineffective. A total of about 80 enemies will appear. When they stop, restart from the checkpoint and repeat the process until you get the achievements. Note: When your health meter reaches zero when bleeding out, your character will fall back then get back up.

Achievements
Complete each achievement to get the allotted gamerscore.

Each time you finish a different skill level, you get free weapons. When you finish the easy level you get all the primary weapons. When you finish the normal level you get all the secondary ones. Last when you finish the hard level you have unlocked all of the weapons. So don't spend all of your money at the end or anything because you get them all for free. XD