The simplest way to do this one is to become familiar with the game.
Beat it a couple of times to understand what classes you should be using to quickly power through the game.
Next step will be to unlock all of the store items that help optimize your best groups skills.
Next save up a LOT of gold hundreds of thousands of gold will be needed for this run to succeed.
You will not be resting unless absolutely necessary.
Buy 200 grindstones to max the level of the blacksmith.
Buy experience potions to level your characters up so that they heal fully rather than resting.
Buy all of the best weapons and armor for your characters.
Do only quests marked with a star (except for the DLC portal).
If you do it perfectly with no resting at all you will beat the game on day 344 that gives you 21 days to mess around with.