[Gimli]

Actually I find that part (cleaning out the outside of the fortress) to be one of the most challenging parts in the game early on. What is a pain about it is the sheer number of foes, and the fact that they have a few casters they are able to hide well behind their melee types (worg riders, orc warriors, and the trolls).

What I suggest doing is to engage them briefly (take a few ranged pot shots), and then run away, almost to the beginning of the map. Let them chase after you and come at you in lesser numbers. And I would hide someone stealthy a little ways before where you're going to make your stand. That guy should then sneak attack any spell casters that show up as they will tend to hang behind the melee forces. Basically you want to thin their numbers and get at their casters.