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.
Showed Carmine the ropes
Successfully defended the hospital
Recovered 5 of 41 collectibles
Completed 1 chapter in Co-op on any difficulty
Mastered the ways of the Active Reload
Escorted Betty to Landown
Team survived 10 Horde waves
Seen the sights with Betty
Watched Ilima sink
Freed Baird from capture
Performed a chainsaw multiple bypass
Discovered the location of the Locust queen
Completed 10 chapters in Co-op on any difficulty
Arrived safely at Mount Kadar Stranded camp
Torched 30 enemies
Bashed 30 Tickers
Won all Training Grounds matches
Defeated the Leviathan
Found Maria
Killed an enemy with every weapon
Activated Jack as homing beacon in Nexus
Killed 30 enemies with your Mulcher turret
Played 1999 rounds in multiplayer
Team survived all 50 Horde waves
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
Defeated Skorge and his Hydra
Found the Locust queen
Killed 10 enemies with a grenade while down but not out
Won a match on every map shipped with the game
Got up close and personal with the Boomshield
Recovered 20 of 41 collectibles
Submitted a spectator photo
Won 10 rounds of Guardian as the leader (not Custom)
Used 10 meatshields to survive
Effectively used grenade mines
Killed 30 enemies raining down death from the sky
Complete all campaign acts on Insane Difficulty
Recover all 41 collectibles (any difficulty)
Win 10 chainsaw duels (any mode)
Perform all 11 unique executions on a downed enemy
Kill 100,000 enemies (any mode)
Win 3 matches of Wingman (not Custom)
Capture 10 meatflags in Submission (not Custom)
Win 3 matches of King of the Hill (not Custom)
Won a match on each of the Flashback Map Pack maps
Complete waves 1 through 10 on all Flashback Map Pack maps in Horde (any difficulty)
Win a Wingman match on each of the 5 Flashback Map Pack maps
Won a ranked or social Annex match on each of the Flashback and Combustible Map Pack maps
Won a match on each of the Combustible Map Pack maps
Survive first 10 Horde waves on Combustible Map Pack maps as a team
Win a Wingman match on each of the 3 Combustible Map Pack maps
Reached level 25 in multiplayer
Reached level 15 in multiplayer
Reached level 5 in multiplayer
Team survived all 50 Horde waves on a Snowblind Map Pack map
Team survived first 10 Horde waves on Snowblind Map Pack maps
Played in the snow and won on all of the Snowblind Map Pack maps
Reached level 50 and played in the snow
Played in the snow on all of the Snowblind Map Pack maps
Reach level 100 and win a match on each of the 4 Snowblind Map Pack maps (not Custom)
Won a match on all of the Dark Corners Map Pack maps
Reached level 50 and got Nowhere in Horde
Played a match on all of the Dark Corners Map Pack maps
Made it through "Road to Ruin" dressed as a Theron Guard
Complete the "Road to Ruin" deleted scene after choosing the guns blazing option
Complete the "Road to Ruin" deleted scene in co-op as either Marcus or Dom
Reach level 8 in multiplayer and complete waves 1 - 10 on the War Machine map in Horde
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
Kill 25 Bloodmounts with a Scorcher while playing Horde on a Dark Corners Map Pack map