Select "Single Player" at the main menu. Select the "Start New Game" option, then press Up(2), Down, Left, Right(2), Down.

Complete each achievement to get the allotted gamerscore.

Complete each achievement to get the allotted gamerscore.

Finish a multiplayer match with 100% accuracy (Minimum of 10 kills).
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This achievement is easier done with a boosting partner. Use your knife to get 10 kills. Since missed knife slashes doesn't count to your accuracy, kill your partner 9 times using the knife then kill him once using a gun.

Perform 300 reloads in any multiplayer mode.
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This is best done with a boosting partner. Grab an automatic rifle or sub-machine gun and fire 1 bullet then reload. Continue doing this until you unlock the achievement. This must be done in a single match only.

Get a triple kill with an explosive during a single Ranked Match - Team Deathmatch.
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Best done with at least 3 other boosting partners. Have them stand together, leave them with critical health then throw a grenade at them.

Accumulate 30 kills with a silencer over Xbox LIVE.
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The kills are cumulative so you don't need to get 30 kills in one match. Its best to use an M16 with silencer to get more kills.

Get 20 kills with a shotgun in a single Ranked Match - Capture The Flag.
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Have a boosting partner and set your game to Capture the Flag mode. Just kill him 20 times using a shotgun to unlock this achievement easily.

Set your game to Demolition mode. Have your boosting partner to plant the bomb then defuse it to unlock this achievement.

Just kill with a headshot and wait the match to finish. You can also do this with a boosting partner while trying to unlock the "Every Bullet Counts" achievement. Just kill him using headshots only.

Achieve 10 kills from a distance greater than 100 feet without a scope in any multiplayer mode.
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Don't use the scope or a gun with a scope to score kills at a distance of more than 100 feet. Have your boosting buddy stand still and kill him until you unlock the achievement.

Achieve 15 kills with a sniper rifle in a single Ranked Match - Deathmatch.
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Set your game to Deathmatch mode and grab a boosting buddy. Kill him 15 times. Remember that the match must be ranked and all kills must be done in a single match.

Achieve 5 headshots in a row without dying using a sniper rifle in a multiplayer match.
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You may unlock this while trying to get the Assassination King achievement with a boosting buddy. Remember that you can only use a sniper rifle as you kill your partner using head shots.

Kill the bomb carrier with an assault rifle 5 times in a single Ranked Match - Demolition.
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If you have a boosting buddy, just let him pick up the bomb then kill him 5 times.

If you are in a real match, just use a sniper or silenced weapon then camp near the bomb. Kill any opponents trying to take it.

Achieve 5 kills in a row without dying with an SMG in a single Ranked Match - Elimination.
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Start a boosting Elimination match then use an SMG (Sub-Machine Gun) to kill your friend(s) to unlock this achievement easily. Otherwise, you'll need to camp and wait for other players that will wander around corners or doors. Remember that if you die at least once without getting 5 kills, you'll need to start a again.

Kill 10 opponents with a machine gun during a single Ranked Match - Team Elimination.
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The kills must be made during a single match only. You can simply boost with friends or just use your machine gun at medium range in crowded areas for maximum efficiency.

Achieve 50 kills using pistols in any multiplayer mode.
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You can do this while you're trying to unlock other achievements. Just boost with your friends and use a pistol at close range to make 50 kills. Its harder to do this in a real match since the pistol is not that effective.

Shoot off all 4 limbs on any combination of players over the course of a multiplayer match.
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You'll need to shoot off the right/left arm and right/left legs of your enemies. The more players the better. Its easier done using a sniper rifle or a shotgun at close range. You'll need to do this during the entire match only.

Kill 50 opponents in Ranked Matches with a knife attack from behind.
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You can do this while trying to unlock the other achievements with a single boosting partner. Simply use the knife to backstab him and get 50 kills.

Accumulate 250 dismemberments in any multiplayer mode.
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This achievement is cumulative so you can do this over the course of several multiplayer matches. Its best to use snipers and shotguns since they can take off limbs easily.

Kill 30 enemy flag carriers in Capture The Flag.
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Almost the same as the "This is my Rifle" achievement, but you'll need to kill more. Just camp near the bomb and use silenced weapons. Alternatively, you can just have a friend carry the bomb while you kill him.

Just commit suicide 5 times and don't participate in the remaining of the match.

Be the most skillful player in a Ranked Match.

You can get this easily by winning a Deathmatch round. A boosting partner will be great as well.

Survived an entire Deathmatch without hurting anyone (minimum of 4 active players).
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In a real match, just choose a good spot to hide. You must not die or wound enemies during the entire game to unlock this achievement.

Killed yourself and an opponent with an explosive 10 times in any multiplayer mode.

Easier done with a boosting partner. Just fire an RPG at point blank range or throw a grenade at your feet.