Specifically for the black forest:

Avoid the pink glowy things, those are greydwarf spawners and sometimes they will spawn a brute or a shaman.

Leave the area at night until you feel confident in fighting the toughest thing there.

Greydwarfs and everything else are attracted to sound. If you chop down a tree or start mining, you will be greeted by the welcoming committee. Sneak is good if you don't want everything running after you.

When one greydwarf aggros, deal with it as soon as possible so you can fight them one at a time instead of in a mob. When in doubt, run. Sometimes they chase for a long time. If you can get them to spread out, you have a chance.

Trolls move faster in the meadows than they do in the forest because they have to walk around stuff you can jump over or squeeze through.

If you see skeletons but there's no tower, there is a crypt somewhere nearby. Upgrade all of your deer armor, or get some troll armor before you go in one. Shields help against archers. The stagbreaker and the club are your friends. A campfire right inside the entrance can be helpful. (You need to go in here to get stuff before you can progress.)

Mobs can not climb vertical cliff edges and are reluctant to fall down them, even if they're not very tall. (Hint: You can make these.)