12 Days of Pandora
To unlock the "12 Days of Pandora" you must make a minimum number of kills with certain weapons or weapon classes. Complete this challenge by getting 12 kills with the assault rifle, 11 kills with the pistol, 10 kills with the shotgun, nine SMG kills, eight kills with the sniper rifle, seven melee kills, six critical hit kills, five explosive kills, four kills by shock, three kills with incendiary weapons, two corrosive kills and one kill with a grenade. Go to the "Challenges" menu to track your progress.
Boss Battles
You will earn several of the secret achievements by defeating bosses as you progress through the story. Kill Sledge for an achievement and Krom for another. Defeat Flynt for another achievement. Lastly, kill the Rakk Hive for the last boss battle secret achievement. You won't miss any of these if you complete the game, as winning these battles is required for story progression.
United We Stand
Get one more secret achievement called "United We Stand" for defeating the four bosses in Section 2 -- Rakk Hive, Krom, Flynt and Sledge -- plus the Vault Boss in a cooperative game. Play with another player on Xbox Live, via system link or on split-screen on one Xbox 360 console to unlock this achievement. The boss battles will be harder, but this is mitigated by having the help of a second player.
Secret Boat
Unlock the last secret achievement by finding a hidden area in Treachers Landing. When you enter this area, access your map and zoom out. Look on the right side of the map for a dock. Make your way to this area and jump on the boat tethered to the dock. This will get you the achievement "You're on a Boat."