Fast to the Finish Achievement
Fast to the Finish10
Completed game within 8 hours.
This achievement is missable!
For many returning players, this achievement will not seem particularly difficult. Ignoring side-quests, maximizing the efficiency of your leveling and sticking only to story-advancement should make it a breeze. If you're new to the series, it's very much possible to follow a walkthrough and still obtain this, though I wouldn't recommend ruining your first playthrough like that. One useful tip for those looking to earn this legitimately is that the game quicksaves your progress at literally every step, so if you are put in a random encounter, you can simply hit the back button to return to the main menu, then press "resume," and you will only be one step behind where you were, and unlikely to face a battle on the same spot as before. In this way you can move around the game with total confidence, even if at 1 hp, because no random encounter will threaten you. Obviously some scripted battles and boss battles cannot be avoided. This will also ensure you have quite the grind to get the "Monster Massacre" achievement if you use this trick often, but can maximize the efficiency of your leveling up.
There is another, far easier way to gain this achievement, however. When you reach the final boss, make sure to save before fighting him. Once you have beaten him, the credits will roll. When they finally finish, tap the screen (when it literally says "The End."), and you will be taken back to the main menu. From here, select New Game, and start as normal. But instead of trying to play through the game again, just roam around the starting area (or the Chaos Shrine for speed's sake), and let your characters get attacked by monsters until they all die. At this point, the credits will roll again, and the achievement will pop, since the game will recognize it as a legitimate ending.
For this method to work, you must select New Game from the main menu directly after watching the credits. You cannot exit the game after earning the achievement for beating the final boss, then try to start a new game; it will not work. But if you don't remember and reset the game somehow, just reload the save from before fighting the final boss, beat him again, and proceed as before.