If it's a single dvergr, backstab them with Skoll and Hati or any other powerful secondary attack. If it's a bunch of dvergr, especially in a tower, you can snipe them with crossbow for nearly guaranteed insta-kills.

Originally posted by knighttemplar1960:

They are weak to fire. If you snipe them with fire arrows before they see you a no star greydwarf will go down in one hit. Except for the shaman and the brute they are also afraid of fire and will run from you if you are holding a lit torch.

I don't see any fire weakness listed on the wiki, I'd stick to crossbow.