Mistlands is always rough, but it's not always wet. There are often sections of it that border mountains or other biomes that have flat areas. But it is best to go ahead and jump up there because you will be safer from the seekers and soldiers if you're not on the ground. If you do get attacked by a lot of seekers, keep looking around that area because there might be something there you need.

Most of the black marble does come from the buildings. You can also get it from giant skeletons. The skulls are especially valuable.