Here is the full list of all 55 Disgaea 2 PC achievements.
Saw Bad Ending 1
Saw the Tink ending
Defeated the Extra Boss "Item God"
Cleared Extra Stages "Cave of Ordeals"
Defeated the Extra Boss "Marjoly"
Obtained Total LV 5,000
Defeated the Extra Boss "Flonne"
Defeated the Extra Boss "Laharl"
Defeated the Extra Boss "Protype Kurtis"
Defeated the Extra Boss "Kurtis"
Obtained LV 1,000!
Cleared 100% of the Dark World stages
Dealt 100,000 damage!
Cleared 50% of the Dark World stages
Cleared 20% of the Dark World stages
Obtained 99 felonies
Defeated the Extra Boss "Priere"
Saw the Laharl ending
Saw the Etna ending
Earned 10,000,000 HL from a single battle
Completed Ch. 4 of Axel's Story "The Dark Hero!"
Unleashed Magichange 2
Performed your first Peta spell
Completed Ch. 1 of Axel's Story "The Evil Rangers"
Performed your first Magichange
Saw the Axel ending
Defeated the Extra Boss "Asagi"
Defeated the Extra Boss "Mid-Boss"
Obtained LV 100!
Completed Final Chapter "The Truth"
Unlocked the Dark World
Defeated the Extra Boss "Prism Ranger"
Obtained 1,000,000 HL!
Completed Ch. 10 "The Two Ninjas"
Performed your first reincarnation
Killed 1,000 enemies!
Completed Ch. 5 "The Promise"
Defeated your first pirate in the Item World
Attended your first trial in the Item World
Killed 100 enemies!
Completed Ch. 1 "The Overlord's Daughter"
Defeated the Extra Boss "Zetta"
Cleared Extra Stage "Evil Academy Special Class!"
Defeated the Extra Boss "Baal"
Defeated the Extra Boss "Pringer X Go"
Defeated all EX bosses!
Dealt 1,000,000 damage!
Dealt 10,000,000 damage!
Dealt 100,000,000 damage!
Obtained LV 9,999!
Obtained Total LV 10,000
Reincarnated 10 times
Obtained the Yoshitsuna!
Entered the Land of Carnage for the first time
Saw Bad Ending 2