Just keep attacking him, when he gets low on health the first few times, he absorbs dragon souls from living dragons to regain health, but on the fourth time he'll run out of dragons and use two shouts to try and escape, but just keep attacking him, eventually you'll get him.

If you have a one hit kill sword DO NOT use it, it will rob him of his chance to absorb a dragon soul and will not allow you to continue. This part is scripted and must happen.

IMO it's very easy to kill him I did it first try

Run into the creek on the left and work your way up and stand on top of the white things sticking out from the gate. Just walk up them until your out of his range and just use magic or your bow. I'm sorry I can't provide to much detail on it. It's been a LONG time since I've played that game, even longer since u fought him lol. I hope I helped in some way or another. Just make sure your out of his range (he will only walk so far from the gate) and attack from a distance.