Hydriss and Assault Commander Jorb
Intrepid Drowned halls is a fairly easy 10 man raid. In order to get the Intrepid: Undertow Achievement you need to kill last boss Hydriss with first boss Assault Commander Jorb still allive.
This video demonstrates the achievement Intrepid: Undertow.
If you have any additional questions feel free to post them in the comments. I can write down a proper strategy summary if requested.