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

That Coulda Gone Better (30 points): Finish the Single Player game on Hard Difficulty.
Ooooh Yeah! (10 points): Kill 5 enemies with one L.E.O. shoulder bash.
Losses (15 points): Make it through to the lair.
One Big, Ugly Motha... (20 points): Defeat the source of it all.
Hit 'N Run (15 points): Kill an enemy by ramming them with the Inferno GX.
Exterminator (15 points): Destroy 100 Pods while piloting the Mantis.
Nanergy! (30 points): Gather 25,000 total salvage.
Martian Drive-By (15 points): Kill 100 enemies while in the Marauder Scout Walker.
Crusader (15 points): Kill 175 enemies while in the L.E.O. exoskeleton.
Salvager (20 points): Find 200 piles of salvage.
Knock, Knock (15 points): Open up the secret entrance.
Things Fall Apart (20 points): Destroy the Water Filtration Plant.
Survival Of The Fittest (15 points): Make it to the surface.
Vanguard (15 points): Escort the convoy.
Secrets Long Buried (10 points): Remove the Seal.
We're Not Alone (10 points): Make it back to civilization.
I'm All You've Got (15 points): Defend the Red Faction.
Old Friends, Older Enemies (15 points): Safely escort Winters through the depths.
Plan B (15 points): Find out how to reach the lair.
Must Go Faster (15 points): Travel to the Marauder homelands.
Weather The Storm (15 points): Destroy the Jamming Devices.
Family Business (20 points): Defeat the Mantis.
Money Well Spent (15 points): Buy out any one Upgrade ring.
All For One, One For All (25 points): Finish a 4 player Infestation game beyond wave 9 without anyone bleeding out.
Soldier (25 points): Finish waves 1 through 10 on any map in Infestation.
Field Surgeon (15 points): Perform Revival 25 times in Infestation.
Boom Goes The Dynamite (10 points): Kill 2 other enemies with a single exploding Berserker.
Crack Shot (10 points): Kill a Wraith before it re-stealths.
Commando (50 points): Finish waves 1 through 20 on any map in Infestation.
Back At Ya! (20 points): Make an enemy kill themselves while shooting at your Shell.
Chronicler (25 points): Listen to 40 Audio Logs.
Cheater! (20 points): Buy a Cheat.
Honorary Mason (75 points): Finish waves 1 through 30 on any map in Infestation.
I Need A Nap (75 points): Finish the Single Player game on Insane Difficulty.
What Is Best In Life? (15 points): Perform melee finishers on 25 Creepers (single player only).
It's All In The Wrist (15 points): Send an enemy at least 30 meters with Impact.
Haymaker (10 points): Kill 5 enemies in one shot with Impact.
Zero G War (20 points): Kill 50 Shockwaved enemies before they hit the ground.
Martian Can Opener (40 points): Buy every Upgrade.
Hold Still (10 points): Kill 6 enemies in one use of Shockwave.
Lock And Load (20 points): Keep Berserk active for at least 21 seconds in one use.
Liftoff (10 points): Send an enemy at least 50 meters with the Magnet Gun.
Martian Matchmaker (10 points): Fire an enemy into another enemy with the Magnet Gun.
In. The. Face! (25 points): Kill one of each enemy type with the Maul.
Bug Hunt (20 points): Finish at least one wave on each map in Infestation.
Catch! (30 points): Use the Magnet Gun to fling debris BACK at a Tentacle.
Unto The Breach (15 points): Gain entry into the Terraformer.

Additionally, there is one secret achievement:

Breathe Easy (30 points): Put an end to the threat, once and for all.