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.
Killed 100,000 enemies
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
Completed all campaign acts on Hardcore Difficulty
Completed all campaign acts on Insane Difficulty
Recovered 5 of 41 collectibles
Recovered 20 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
Killed 30 enemies raining down death from the sky
Effectively used grenade mines
Got up close and personal with the Boomshield
Torched 30 enemies
Mastered the ways of the Active Reload
Bashed 30 Tickers
Killed an enemy with every weapon
Executioner with style
Won all Training Grounds matches
Killed 10 enemies with a grenade while down but not out
Played 1999 rounds in multiplayer
Won a match on every map shipped with the game
Won 3 matches of Wingman (not Custom)
Captured 10 meatflags (not Custom)
Won 3 matches of King of the Hill (not Custom)
Won 10 rounds of Guardian as the leader (not Custom)
Team survived all 50 Horde waves
Team survived 10 Horde waves
Submitted a spectator photo
Completed 10 chapters in Co-op on any difficulty
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 Wingman match on each of the Combustible Map Pack maps
Won a ranked or social Annex match on each of the Flashback and 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
Reached level 25 in multiplayer
Reached level 15 in multiplayer
Reached level 5 in multiplayer
Became an ultimate Gearhead at level 100
Team survived all 50 Horde waves on a Snowblind Map Pack map
Team survived first 10 Horde waves on Snowblind Map Pack maps
Reached level 50 and played in the snow
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
Reached level 50 and got Nowhere in Horde
Reached level 40 and Memorialized the Horde
Reached level 30 and paid your Horde respects
Cooked up some Bloodmount BBQ with 25 roastings
Reached level 20 and stopped at the Horde Way Station
Reached level 10 and hit the Horde Highway
Reached level 15 and found some Horde Sanctuary
Reached level 8 and rocked Horde old school
Made it through "Road to Ruin" dressed as a Theron Guard
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" with a friend
Made it through "Road to Ruin" with nothing but vengeance on your mind