Here is the full list of all 55 Disgaea 2 PC achievements.
Defeated your first pirate in the Item World
Completed Ch. 5 "The Promise"
Attended your first trial in the Item World
Killed 100 enemies!
Completed Ch. 1 "The Overlord's Daughter"
Completed Ch. 10 "The Two Ninjas"
Completed Final Chapter "The Truth"
Completed Ch. 1 of Axel's Story "The Evil Rangers"
Completed Ch. 4 of Axel's Story "The Dark Hero!"
Unlocked the Dark World
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 "Asagi"
Defeated the Extra Boss "Prism Ranger"
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!
Killed 1,000 enemies!
Obtained 1,000,000 HL!
Earned 10,000,000 HL from a single battle
Obtained LV 100!
Obtained LV 1,000!
Obtained LV 9,999!
Obtained Total LV 5,000
Obtained Total LV 10,000
Performed your first reincarnation
Reincarnated 10 times
Obtained the Yoshitsuna!
Performed your first Magichange
Unleashed Magichange 2
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