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
Escorted Betty to Landown
Seen the sights with Betty
Watched Ilima sink
Freed Baird from capture
Performed a chainsaw multiple bypass
Discovered the location of the Locust queen
Arrived safely at Mount Kadar Stranded camp
Defeated the Leviathan
Found Maria
Activated Jack as homing beacon in Nexus
Found the Locust queen
Defeated Skorge and his Hydra
Successfully defended COG headquarters
Captured a Brumak
Defeated the Lambent Brumak
Completed all campaign acts on Casual Difficulty
Completed all campaign acts on Normal Difficulty
Recovered 5 of 41 collectibles
Recovered all 41 collectibles
Completed 1 chapter in Co-op on any difficulty
Completed all acts in Co-op on any difficulty
Won 10 chainsaw duels
Used 10 meatshields to survive
Killed 30 enemies with your Mulcher turret
Got up close and personal with the Boomshield
Torched 30 enemies
Mastered the ways of the Active Reload
Killed an enemy with every weapon
Executioner with style
Killed 100,000 enemies
Won all Training Grounds matches
Played 1999 rounds in multiplayer
Won a match on every map shipped with the game
Won 3 matches of Wingman (not Custom)
Won 10 rounds of Guardian as the leader (not Custom)
Completed all campaign acts on Hardcore Difficulty
Completed all campaign acts on Insane Difficulty
Won 3 matches of King of the Hill (not Custom)
Team survived all 50 Horde waves
Killed 10 enemies with a grenade while down but not out
Team survived 10 Horde waves
Effectively used grenade mines
Recovered 20 of 41 collectibles
Submitted a spectator photo
Killed 30 enemies raining down death from the sky
Bashed 30 Tickers
Completed 10 chapters in Co-op on any difficulty
Capture 10 meatflags in Submission (not Custom)
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 Flashback Map Pack maps
Won a match on each of the Combustible 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
Win an Annex match on each Flashback and Combustible Map Pack map (not Custom)
Reached level 5 in multiplayer
Reached level 25 in multiplayer
Reached level 15 in multiplayer
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
Team survived first 10 Horde waves on Snowblind Map Pack maps
Reached level 50 and played in the snow
Team survived all 50 Horde waves on a Snowblind Map Pack map
Reach level 100 and win a match on each of the 4 Snowblind Map Pack maps (not Custom)
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 nothing but vengeance on your mind
Made it through "Road to Ruin" with a friend
Reached level 8 and rocked Horde old school
Reached level 10 and hit the Horde Highway
Reached level 20 and stopped at the Horde Way Station
Reached level 30 and paid your Horde respects
Cooked up some Bloodmount BBQ with 25 roastings
Reached level 15 and found some Horde Sanctuary
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