me and my guild have been trying to kill the rotscale for over 2 weeks

, we changed builds and tried different party's and different tactics but we can never kill the Rotscale, we managed to kill all the other dragons but not him. We are trying to kill him mainly because now its some sort of challenge which we want overleap! but we need some help because were finding it extremly difficult.
Any1 knows any build or any link from which we can find that that we seek? thx in advance