This game offers four different achievements for difficulty:
Moderate, Hard, Evil & Chaos.

No, you don't have to play the whole game to unlock these achievements.
Tip: DO NOT attempt this achievement after the Zhonecage optional dungeon. After level 10, it buffs the final boss for another achievement and you don't want to make it harder than it already is.

Go to the point of no return in Ghardia Shaft .
Before entering the final battle, go to the Settings menu and change it to either Moderate, Hard, Evil or Chaos.

Once you have selected the difficulty, you will be able to take on the final boss, which will unlock the Achievement upon completion. My party looked like this:



Tip: If you find the Evil or Chaos Achievements too hard, you can try activating items in the Grade Shop, which can also be found in the Settings tab.
Tip 2: If you have Double Crit, 5x Damage enabled and are still having trouble killing the boss on the selected difficulty, try farming the Hourglass item.

I collected 30 of them and entered the final battle. Have another member of your team use the Hourglass every time it runs out and deal damage with Asbel and you're good to go. Depending on the damage, you shouldn't need more than 20 hourglasses.

https://i.imgur.com/iKE04Oy.png

If you found this guide helpful, please feel free to like, favourite and award it, and if you feel generous:

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It's not necessary though, I'm just glad I could help <3