Pro-tip: Abuse the gambit system.
In the bowels of the Stilshrine of Miriam, there's a rare enemy that will occasionally spawn named Negalmuur. You have to first kill all enemies in the area called "Ward of the Sword King." Wait 30 minutes and then re-enter the area; Negalmuur will spawn in the T-shaped hallway. Negalmuur spawns an infinite number of ghasts, which will be your target.
Set up your party's gambits as follows:
Self - Haste
Ally: HP < 70% - Cura
Ally: Any - Raise
Ally: Any - Esuna
Ally: Party Leader - Remedy
Foe: HP < 3,000 - Attack
Self - Charge (Optional: Self: MP < 10% - Charge)
Character 2:
Self - Haste
Ally: HP < 70% - Cura
Ally: Any - Raise
Ally: Any - Esuna
Ally: Party Leader - Remedy
Self - Decoy
Foe: HP < 3,000 - Attack
Self - Charge (Optional: Self: MP < 10% - Charge)
Character 3:
Self - Haste
Ally: HP < 70% - Cura
Ally: Any - Raise
Ally: Any - Esuna
Ally: Party Leader - Remedy
Self - Decoy
Foe: HP < 3,000 - Attack
Self - Charge (Optional: Self: MP < 10% - Charge)
You will need a pretty decent supply on remedies to counter the doom status effect, but esuna will take care of everything else. The ghasts he spawns have under 3,000 HP, so your party will only target them as Negalmuur has over 18,000 HP. Let this run while you go watch TV or something.
Decoy isn't really necessary. It will just make things go smoothly as it will increase the likelihood that your party leader stays alive (thus not pausing the game and forcing you to pick another character). Beyond a certain point, you won't really need to worry about dying anyway. The longer you do it, the safer it becomes. You might want to do some baby-sitting until your characters reach the upper 30's or early 40's. Being up-to-date on equipment also helps.