Ok, all together!
Disgusing as a Dunlending is sadly a bit of a non-starter. You need to look horribly primitive to pass as one and they don't wear much- kinda like the Scots from the Braveheart era. Even if you had the right skills, it would be extremely hard (and impossible for anyone with a touch of Elf about them).
A surprise attack is possible but tricky as it would mean walking into the encampment without being spotted. You could surprise them at the point where you start charging in, but then much depends on how fast you run in.
Lacharon can indeed try to snipe but as he went for the easy observation he does not have a perfect read on their outside sentries so that is not risk-free.
If you have metal armour, you can wear it for this but bear in mind it slows you down.
Ok, idea.
What if we do the sniping plan, Lacharon snipes the sentries, from slight distance, enough so that the Dunlendings have to come out of their encampment to engage him. However, all of our heavy melee fighters are hidden somewhere between the encampment and Lacharon. Then when they come out of their encampment, we hit them hard.
I can help with the hiding, because I can cast Shade and be useful.
Lets kill them!