Full list of all 79 Gears of War 2 achievements worth 1,750 gamerscore. It takes around 80-100 hours to unlock all of the achievements in the base game.
The base game contains 50 achievements worth 1,000 Gamerscore, and there are 5 DLC packs containing 29 achievements worth 750 Gamerscore.
Completed all campaign acts on Insane Difficulty
Team survived all 50 Horde waves
Played 1999 rounds in multiplayer
Recovered 20 of 41 collectibles
Bashed 30 Tickers
Won 10 chainsaw duels
Team survived 10 Horde waves
Killed 10 enemies with a grenade while down but not out
Effectively used grenade mines
Used 10 meatshields to survive
Torched 30 enemies
Won a match on every map shipped with the game
Killed an enemy with every weapon
Won all Training Grounds matches
Got up close and personal with the Boomshield
Executioner with style
Defeated the Lambent Brumak
Completed all acts in Co-op on any difficulty
Completed all campaign acts on Casual Difficulty
Completed all campaign acts on Normal Difficulty
Completed all campaign acts on Hardcore Difficulty
Captured a Brumak
Successfully defended COG headquarters
Killed 30 enemies raining down death from the sky
Killed 30 enemies with your Mulcher turret
Defeated Skorge and his Hydra
Found the Locust queen
Activated Jack as homing beacon in Nexus
Found Maria
Defeated the Leviathan
Arrived safely at Mount Kadar Stranded camp
Discovered the location of the Locust queen
Performed a chainsaw multiple bypass
Freed Baird from capture
Completed 10 chapters in Co-op on any difficulty
Watched Ilima sink
Submitted a spectator photo
Seen the sights with Betty
Mastered the ways of the Active Reload
Recovered 5 of 41 collectibles
Escorted Betty to Landown
Successfully defended the hospital
Completed 1 chapter in Co-op on any difficulty
Showed Carmine the ropes
Recover all 41 collectibles (any difficulty)
Kill 100,000 enemies (any mode)
Win 3 matches of Wingman (not Custom)
Capture 10 meatflags in Submission (not Custom)
Win 10 rounds of Guardian as the leader (not Custom)
Win 3 matches of King of the Hill (not Custom)
Won a match on each of the Flashback Map Pack maps
Survived first 10 Horde waves on Flashback Map Pack maps as a team
Win a Wingman match on each of the 5 Flashback Map Pack maps
Team survived first 10 Horde waves on Combustible Map Pack maps
Won a match on each of the Combustible Map Pack maps
Win a Wingman match on each of the 3 Combustible Map Pack maps
Win an Annex match on each Flashback and Combustible Map Pack map (not Custom)
Reached level 15 in multiplayer
Reached level 5 in multiplayer
Earn enough multiplayer experience to reach level 25 (not Custom)
Played in the snow on all of the Snowblind Map Pack maps
Played in the snow and won on all of the Snowblind Map Pack maps
Reach level 50 and play a match on each of the 4 Snowblind Map Pack maps (not Custom)
Reach level 100 and win a match on each of the 4 Snowblind Map Pack maps (not Custom)
Complete waves 1 through 10 on all Snowblind Map Pack maps in Horde (any difficulty)
Complete waves 1 through 50 on any Snowblind Map Pack map in Horde (any difficulty)
Played a match on all of the Dark Corners Map Pack maps
Won a match on all of the Dark Corners Map Pack maps
Reached level 8 and rocked Horde old school
Made it through "Road to Ruin" dressed as a Theron Guard
Made it through "Road to Ruin" with a friend
Made it through "Road to Ruin" with nothing but vengeance on your mind
Reach level 10 in multiplayer and complete waves 1 - 10 on the Highway map in Horde
Reach level 15 in multiplayer and complete waves 1 - 20 on the Sanctuary map in Horde
Reach level 20 in multiplayer and complete waves 1 - 20 on the Way Station map in Horde
Reach level 30 in multiplayer and complete waves 1 - 30 on the Allfathers Garden map in Horde
Reach level 40 in multiplayer and complete waves 1 - 40 on the Memorial map in Horde
Reach level 50 in multiplayer and complete waves 1 - 50 on the Nowhere map in Horde
Kill 25 Bloodmounts with a Scorcher while playing Horde on a Dark Corners Map Pack map