You'll fight the Draug near the end of Act 2 on both, Ioverth's and Roche's paths. The fight begins with a smaller enemy, that will yield after you get its health under 50%.
The Draug battle would be long and hard with arrows and fire raining from the sky and the Draug rushing you and using very long reaching melee attacks. It also has a shield that you have to first deplete if you attack head on. There however is an alternative.
Prepare by using any enhancements that give you more basic sword attack damage, having 2nd level of "Footwork" talent of Swordmanship tree is recommended. None of the signs are needed.
The Draug turns very slowly after rushing and the idea is to make him rush into a wall. Any point of the level will do. Just be close enough to make him rush, being too far away will call for arrows or fireballs.
Once he rushes, get out of the way and right behind its back. If you keep smashing it without stopping, it won't turn around if it stopped because of some obstruction, like the level borders. Use the more powerful attack with X. Should your attack for some reason fail to drain its life completely, repeat. Works great on all levels, including Dark. The same trick can be used to beat Endreka queens.
Here's a video of it going down 15 seconds from the fight start: