From the Storehouse Firs Floor Site of Grace
Turn left towards the room with black flame sorcerors
take the elevator down until you come across the pot enemies again
keep going forward until you reach the hall where of them is standing infront of the item at the table .
Or take the West Rampart site of grace and backtrack to the room.
that pot enemy and the monster inside him will be one of the sturdier and harder ones. You'll know