Unlockables - Complete the following tasks to earn the desired achievement. Note that these only work on the PC version; they will NOT work in the Xbox version.

Unlockable How to Unlock

(Campaign level goes here) (30 each) Complete each level (seperate achievements) on Normal, Heroic, or Legendary. There are 13 levels (not counting tutorial) for a total of 390 points.
Air Traffic Control (20) Blow up the Banshee in flight while manned, with grenades or the rocket launcher
Assassin (10) Melee an opponent from behind

Bonecracker (10) Hit and kill an opponent from behind

Carjacking (5) Steal an occupied vehicle from an opponent and take it over
Counterpoint (5) Kill the sword carrier

Decorated Soldier (10) Get awarded at least 8 different medals in one non-team game
Demon (25) Complete any level without dying. The tutorial level does not count.
Double Kill (20) Kill 2 opponents within 4 seconds

Go Ape Shiv (20) Kill an enraged, berserk Brute by melee

Hero (50) Complete the game on Heroic difficulty
Hired Gun (20) If you know someone in the game who has completed the game on Legendary, then kill him.

Killing Spree (20) Kill 5 opponents in a row in the same game, without dying
Killtacular (20) 4 opponents
King of the Scrab (20) Aquire the Scrab gun on the Metropolis level

Legend (50) Complete the game on Legendary difficulty

Ninja (20) In a non-team game, kill 5 people by melee, from behind

Rainman (25) Play every game variant, with at least 3 other players, until the end of the game
Roadkill (10) Run over, hit and kill an opponent with a vehicle
Running Riot (20) 10 opponents, same condition

Skewer Stopper (20) Kill the sword carrier, after he/she has killed 5 or more opponents in a row with the sword
Slient but Deadly (20) Kill 7 enemies in a row without being spotted
Stick It (5) Stick an opponent (MP) with a plasma grenade and blow them up

Triple Kill (20) 3 opponents

Vigilante (10) End someone's killing spree.
Violent Cartographer (25) Play every default map and stay in the game until it's over

Warrior (100) Complete the game on Normal difficulty