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 acts in Co-op on any difficulty
Completed all campaign acts on Normal Difficulty
Completed all campaign acts on Hardcore Difficulty
Completed all campaign acts on Insane Difficulty
Recovered 20 of 41 collectibles
Completed 10 chapters in Co-op on any difficulty
Won 10 rounds of Guardian as the leader (not Custom)
Activated Jack as homing beacon in Nexus
Killed 30 enemies raining down death from the sky
Executioner with style
Effectively used grenade mines
Used 10 meatshields to survive
Bashed 30 Tickers
Team survived 10 Horde waves
Won a match on every map shipped with the game
Won 10 chainsaw duels
Killed an enemy with every weapon
Defeated the Lambent Brumak
Completed all campaign acts on Casual Difficulty
Captured a Brumak
Killed 30 enemies with your Mulcher turret
Got up close and personal with the Boomshield
Successfully defended COG headquarters
Defeated Skorge and his Hydra
Found the Locust queen
Showed Carmine the ropes
Submitted a spectator photo
Found Maria
Torched 30 enemies
Defeated the Leviathan
Arrived safely at Mount Kadar Stranded camp
Won all Training Grounds matches
Mastered the ways of the Active Reload
Discovered the location of the Locust queen
Performed a chainsaw multiple bypass
Recovered 5 of 41 collectibles
Freed Baird from capture
Watched Ilima sink
Seen the sights with Betty
Escorted Betty to Landown
Completed 1 chapter in Co-op on any difficulty
Successfully defended the hospital
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 3 matches of King of the Hill (not Custom)
Kill 10 enemies with a grenade while down but not out (any mode)
Play 1999 rounds of multiplayer (any mode)
Survive all 50 waves of Horde (any difficulty, any map)
Complete waves 1 through 10 on all Flashback Map Pack maps in Horde (any difficulty)
Win a multiplayer match on each of the 5 Flashback Map Pack maps (any mode except Wingman)
Win a Wingman match on each of the 5 Flashback Map Pack maps
Survive first 10 Horde waves on Combustible Map Pack maps as a team
Win a multiplayer match on each of the 3 Combustible Map Pack maps (any mode except Wingman)
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 25 in multiplayer
Reached level 15 in multiplayer
Reached level 5 in multiplayer
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)
Play a multiplayer match on each of the 4 Snowblind Map Pack maps (any mode)
Win a multiplayer match on each of the 4 Snowblind Map Pack maps (any mode)
Made it through "Road to Ruin" with nothing but vengeance on your mind
Without being detected, complete the "Road to Ruin" deleted scene after choosing the stealth 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
Reach level 50 in multiplayer and complete waves 1 - 50 on the Nowhere map in Horde
Play a multiplayer match on each of the 7 Dark Corners Map Pack maps (any mode)
Win a multiplayer match on each of the 7 Dark Corners Map Pack maps (any mode)
Kill 25 Bloodmounts with a Scorcher while playing Horde on a Dark Corners Map Pack map