I did it by sending in a Focus Sashed Bibarel and yawning on the first turn, then switching into Cofagrigus and doing power split, then I spam shadow balls (got some spdef drops) till he waked up, did will-o-wisp, switched out to some random, healed Cofragigus, wait till death, send Cofragigus + power split. Rinse and repeat + will-o-wisping when he would full restore, then I spammed hex and I won. I also had Sharpedo in the back w/ aqua jet and a revive just in case.