Here is the full list of all 28 Peggle 2 achievements worth 400 gamerscore. It takes around 25-30 hours to unlock all of the achievements on Xbox 360.
Completed 180 Optional Objectives
Hit 300 green pegs
Completed 150 Optional Objectives
Completed every Celestial Realm trial
Completed every trial in Gravely Grove
Completed every trial in Gnorman's Gneighborhood
Completed every trial in Winter Blunderland
Completed every trial in Mountain Highs
Completed 120 Optional Objectives
Hit 5000 orange pegs
Completed every trial in the Peggle Institute
Completed 90 Optional Objectives
Hit 10,000 blue pegs
Won a level with at least 12 balls left
Completed every Celestial Realm level
Completed 60 Optional Objectives
Completed every level in Gravely Grove
Completed every level in Gnorman's Gneighborhood
Completed every level in Winter Blunderland
Completed 30 Optional Objectives
Won a level with the cool clear style shot
Earned 3 free balls on one shot
Completed every level in Mountain Highs
Completed every level in the Peggle Institute
Beat your first ace score
Cleared all pegs in a level
Got 75,000 styleshot points in a shot
Cleared all orange pegs in a level