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.
Team survived 10 Horde waves
Captured 10 meatflags (not Custom)
Played 1999 rounds in multiplayer
Won 3 matches of King of the Hill (not Custom)
Won all Training Grounds matches
Team survived all 50 Horde waves
Won 10 chainsaw duels
Recovered all 41 collectibles
Completed all campaign acts on Insane Difficulty
Killed 30 enemies raining down death from the sky
Killed 30 enemies with your Mulcher turret
Effectively used grenade mines
Killed an enemy with every weapon
Submitted a spectator photo
Won a match on every map shipped with the game
Bashed 30 Tickers
Completed all campaign acts on Normal Difficulty
Completed all campaign acts on Hardcore Difficulty
Completed all acts in Co-op on any difficulty
Recovered 20 of 41 collectibles
Defeated the Lambent Brumak
Completed all campaign acts on Casual Difficulty
Captured a Brumak
Successfully defended COG headquarters
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
Torched 30 enemies
Performed a chainsaw multiple bypass
Completed 10 chapters in Co-op on any difficulty
Freed Baird from capture
Watched Ilima sink
Recovered 5 of 41 collectibles
Seen the sights with Betty
Escorted Betty to Landown
Mastered the ways of the Active Reload
Successfully defended the hospital
Completed 1 chapter in Co-op on any difficulty
Showed Carmine the ropes
Use a meatshield to save your life 10 times (any mode)
Melee 10 enemies down with the Boomshield equipped (any mode)
Perform all 11 unique executions on a downed enemy
Kill 100,000 enemies (any mode)
Win 3 matches of Wingman (not Custom)
Win 10 rounds of Guardian as the leader (not Custom)
Kill 10 enemies with a grenade while down but not out (any mode)
Won a Wingman match on each of the Flashback Map Pack maps
Survived first 10 Horde waves on Flashback Map Pack maps as a team
Won a match on each of the Flashback Map Pack maps
Won a Wingman match on each of the Combustible 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 an Annex match on each Flashback and Combustible Map Pack map (not Custom)
Reached level 25 in multiplayer
Reached level 15 in multiplayer
Reached level 5 in multiplayer
Reached level 50 and played in the snow
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 on all of the Snowblind Map Pack maps
Played in the snow and won 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)
Reached level 50 and got Nowhere in Horde
Reached level 40 and Memorialized the Horde
Reached level 30 and paid your Horde respects
Reached level 20 and stopped at the Horde Way Station
Reached level 15 and found some Horde Sanctuary
Reached level 8 and rocked Horde old school
Reached level 10 and hit the Horde Highway
Played a match on all of the Dark Corners Map Pack maps
Won a match on all of the Dark Corners Map Pack maps
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
Kill 25 Bloodmounts with a Scorcher while playing Horde on a Dark Corners Map Pack map