Maybe you mean mission #4 "Remnants of the Horde". Here's a thread about this mission:
http://forum.paradoxplaza.com/forum/...d.php?t=405757
I think you need to reach the hexer with your troops; at least that's what I did. If you can lure the orcs guarding his fortress into the river, it will be much easier to kill them. I didn't go for the dragon which saved me enough time for a gold victory, but on the other hand the dragon could be usuful for the missions to come.
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