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