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.

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.

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

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

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

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.

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

Iraq

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.

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.

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

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.

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.

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

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.

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.

Easy "Big Boom! Big Bada Boom!" achievement - 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.

Easy "Fear Is The Mind Killer" achievement- 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.

Easy "Fission Mailed" 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.

Easy "If I Were A Rich Man" achievement - Use the "Easy money in China" trick to get the "If I Were A Rich Man" achievement.

Easy "One Shot. One Kill." achievement - 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.

Accomplish the indicated achievement to get the corresponding number of Gamerscore points:

Elite PMC (50 points): Complete all missions as a Professional.
My Virtual Friend (35 points): Complete all missions with the Partner AI.
Say Hello to my Little Friends (45 points): Purchase the MP7, M134, DAO-X Protecta, MGL MK-X, and the M107 Light Fifty.
Man of Many Masks (50 points): Purchase every mask variant.
If I Were a Rich Man (40 points): Earn one million dollars in total over the course of campaign mode.
Fear is The Mind Killer (45 points): Spend 1 minute straight at full Aggro.
Starting a Riot (30 points): Kill 50 enemies total in Co-op Riot Shield.
Two Eyes are Better Than One (35 points): Kill 5 enemies total using Co-op Snipe.
Seven-six-two Millimeter (20 points): Kill 250 enemies total using Assault Rifles.
This is my Boom Stick! (20 points): Kill 250 enemies total using Shotguns.
Spray and Pray (20 points): Kill 250 enemies total using Machine Guns.
Dead Man's Hand (20 points): Kill 150 enemies total using Handguns.
Alright, Who Wants Some? (20 points): Kill 250 enemies total using SMGs.
One Shot. One Kill. (20 points): Kill 100 enemies total using Sniper Rifles.
Big Boom! Big Bada Boom! (20 points): Kill 25 enemies total using RPGs.
Flip You. Flip You For Real (20 points): Kill 50 enemies total using the Melee Attack.
Fission Mailed (20 points): Kill 25 Martyrs by shooting the Bomb Packs on their chests.
If It Bleeds, We Can Kill It (30 points): Kill 25 Heavy Armor enemies.
Field Medic (30 points): Drag and heal your partner 25 times.
Boots on the Ground (10 points): Finish playing a versus ranked match.
The Devil's in the Details (20 points): Complete 20 minor objectives in a ranked match.
Surviving The Game (25 points): Complete a ranked match without ever needing to be revived.
Retirement Savings Plan (40 points): Earn one billion dollars in total in ranked matches.
Out of Debt (15 points): Heal your partner for the first time. Training does not count.
One Gun is Enough (35 points): Win a ranked match without purchasing any additional weapons.
Beast with Two Fronts (20 points): Kill 50 enemies total in Back-to-Back.
Running Man (30 points): Kill 75 enemies while in the stealth mode of Overkill.
L'Abattoir (30 points): Kill 100 enemies while in the power mode of Overkill.
Weapon Specialist (15 points): Purchase 3 weapons.
Stonewall (30 points): Kill 30 enemies using the Riot Shield melee attack
My Kind of Case! (40 points): Collect all of the information cases in the entire game.

Additionally, there are six secret achievements:

Mission 1 Complete (30 points): Completed Mission 1 as Contractor or higher.
Mission 2 Complete (30 points): Completed Mission 2 as Contractor or higher.
Mission 3 Complete (15 points): Completed Mission 3 as Contractor or higher.
Mission 4 Complete (15 points): Completed Mission 4 as Contractor or higher.
Mission 5 Complete (15 points): Completed Mission 5 as Contractor or higher.
Mission 6 Complete (15 points): Completed Mission 6 as Contractor or higher.