The ones above are all great, but it still takes too long to kill them that way. What I use is the first level Marked for Death shout found in the dark brotherhood sancurary. Since its recharge time is short, you can use it 3-4 times and their heal drop by alot. Even works on Elder and Ancient dragons. By using a 105 bow, I was only able to take out 1mm of health, but using the shout a few times, my bow can take out an ancient dragon with only 5-6+ hits with a 105 ebony bow.

The other answers are all great, but it still takes too long to kill them that way. What I use is the first level Marked for Death shout found in the dark brotherhood sancurary. Since its recharge time is short, you can use it 3-4 times and their heal drop by a lot. Even works on Elder and Ancient dragons. By using a 105 bow, I was only able to take out 1mm of health, but using the shout a few times, my bow can take out an ancient dragon with only 5-6+ hits with a 105 ebony bow.