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