The achievements for completing the chapter on lower difficulties will unlock when you complete the DLC on Paladin difficulty.
There are two levels in the chapter in which you’ll be fighting a boss called the Forgotten One. He is not to be underestimated as he is one of the hardest boss in the game.
It is highly recommended to fully upgrade your Light and Shadow magic gauges (by finding all the gems in the game) and to have a full health bar before attempting both levels (load Chapter 5-2 to quickly refill you magic gauges and your health bar). For the best tips on how to win your fight against the Forgotten One, please follow this great guide found on PST HERE.