Like most bosses, use your surroundings to your advantage. With Ned, it's best to use a Revolver or Sniper Rifle and pick him off from across the room, opposite from wherever he is (see video below at 2:05). If you are a much higher level, you can also just shot him down with a Shotgun and that should do the trick.
Credit for video goes to steeldragonz.
Happy Hunting :)