Poison resist mead is not required but it sure does help.

When dealing with archers the shield will serve you well. (or for dealing with anything, for that matter.) Advance while blocking and take them out with a mace. If they're in a tough spot, stand behind a tree and they will come to you.

Stay out of the water as much as possible. Leeches aren't hard to kill but they can sneak up on you.

Try to only explore the swamp during the day, to locate those crypts, and clear them at night.