After clearing the Aleroth catacombs I was strong enough that the orc drummer could not hurt me, but I could not do damage fast enough to kill it. I simply left it, and came back later when I was stronger and had better equipment (actually I forgot about him, and simply ran across him again while I was doing something else).
Concentrate on the other orcs, and maybe run away to draw them after you and away from the drummer's help temporarily.
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