I only experimented this on a thin patch of swamp, but felling down ancient trees on the boundary between swamps and (e.g.) black forest attracts locals and you can fight them in the other biome (prepare a campfire).
Another twisted way is to attract a strong troll (by felling trees), and let the two peoples settle their matter.
Whatever, this region of swamp becomes more sane for some time.

Btw, in troll armor, basic draugar are easily taken one by one, but elite ones are better fought from afar.