A Serene Air Airbus A330-200, registration AP-BNE performing flight ER-504 from Karachi to Islamabad (Pakistan), had departed Karachi's runway 25L and was enroute at FL330 about 180nm northnortheast of Karachi when the crew needed to shut the left hand engine (CF6) down and returned to Karachi. The aircraft drifted down to FL160 and landed safely on runway 25L about 65 minutes after departure.