Solo'd as a 35 rogue, not twinked. On the eastern most side of the island there is a lone Defias. Kill him, then wait for Mathers to stroll by on his usual patrol route. Hit him, back down the slope a bit then wipe him out!
e doesn't run away like a lot of humanoid mobs. Stand up fighter until you lay him down!!

As you're looking at your map of dustwallow marsh, you'll see three islands of about equal size just off the coast there??

Well its the bottom of the 3, closest to Theramore Isle

Co-ords given fall right in the middle of it.

Hope that helps!