I am playing a level 27 thief with the second rank of nightingale equipment + glass weapons, and dustman's cairn was hard! I had already cleared it with a mage and a worrier but only just realized how difficult it is for a thief, the silver hand all had orkish bows and dwarvern arrows. And I was knee-deep in scouge's and waights by the end of the last fight. anyway cutting to the chase, and tips on a theif with quite bad armou (not got arcane blacksmith or any smithing perks)

Well it depends on your build and weapons, i have a thief one H/Archer with lots of points in health without sacrificing stamina much (mostly have amulets/ rings that buff it up a lot) and never had any trouble.