Not too often I see people in Magincia Fel on Origin. The few times I've seen em, they've had nothing of value in their pack. The biggest problems are the constant fire fields being thrown around everywhere.
You're best bet is to snoop em and hope they walk away from you (keeping the fire away). As long as they are within 15 tiles or so, their pack will stay open as long as you don't move (you can shadow jump though and it will stay open). Just wait for something to drop, shadow jump, steal and run.
The only other option would be to sit by the bank in Magincia and hope that's the bank they are using to drop off their cursed arties. If they bring one of the new arites to the bank before insuring it, they deserve to have that taken! This is a rather boring option. Though patience is the key to successful thieving.
Origin Thief