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