The special rooms, which are the ones which have a question mark on the map, regenerate breakable items when you quit to the main menu. Because of this you can farm infinite gold and health/magic which makes the Thanatophobia achievement very easy to obtain. The process is as follows:
1) Explore a bit of the castle until you have several special rooms accessible.
2) Quit to the main menu.
3) Go back into the game and use the warp to jump quickly to each special room.
4) Quit to the main menu and rinse and repeat.
Because you can farm infinite gold you should be able to level up your second or third character enough to reach and beat all bosses fairly easily. You can also use this method to refill your health and magic so you can safely clear out the castle room by room.
Every so often you should exit out of the game and make a backup just in case you die
. Even though you can get back full health any time you want it is very easy to die at level 0 once you start exploring further in the castle.
This is not the quickest method as the farming is a bit of a grind but it guarantees that anyone should be able to achieve this achievement.
You have to do this on a brand new save slot, Play through 2 will not work
Video credit goes to hipo5 on PSNProfiles
Exploit Credit goes to Throw7 on PSNProfiles