Infinite Grenades - You'll get it when You complete the game on any difficulty.
Double Health Regeneration Rate - Complete the game on any difficulty.
Infinite Ammo - Complete the game on any difficulty.
When You complete each achievement You'll get the allotted gamerscore.
Defender (20) - Help defend the barricade in New York city on Normal or higher difficulty
Dolly Madison Award (20) - Save the Constitution from being burned down on Normal or higher Difficulty
Exploding Monkey (20) - Kill at least Four enemies with one explosion on any difficulty level
Environmentalism (15) - Complete 40 environmental or grapple kills on any difficulty
MVP (15) - Win an 8-player ranked or player match in Team Deathmatch mode with the highest score
pwnt (15) - Finish an 8-player ranked or player match in Deathmatch mode with the highest number of kills
World Tour (15) - Finish a multiplayer game on every map in every mode
Combat Experience (10) - Kill 50 enemies in the campaign on any difficulty
One in the Chamber (10) - Kill a player in a ranked match with the last bullet in a pistol clip
Sharpshooter (10) - Kill all the paratroopers as you make your way down the skyscraper on any difficulty
First Time for Everything (5) - Finish a multiplayer game.
Gravity is a Harsh Mistress (5) - Toss a paratrooper onto the streets below New York on any Difficulty
Next Time Take the Stairs (5) - Fall to your death during a multiplayer game
Reservation for One (5) - Make your way down to Hotal Langteau on Normal or higher difficulty
Swirly Time! (5) - Drown a German in a toilet in the Tower of London on any Difficulty
TV Repairs (5) - Smash an enemy's head through a TV in the White House on any Difficulty