After reaching the normal ending, watch / skip the credits and save the game when prompted. Load that save game again and you'll find yourself right before the boss battle. Beat the boss, then select to have Darius dish out the final blow when prompted. This will open up a few more quests that lead to the true ending.
This achievement unlocks when you reach the actual boss fight of the true ending. You'll get another achievement for winning the fight and reaching the true ending.