Mace is going to work best against bonemass. Some may suggest the iron sledge instead, though I'm not a fan. Ideally though, I think you should make the weapon you've been using the most up to this point for the built up skill.

In the end, every weapon is going to be good enough in any situation with enough player skill. And there is (usually) enough iron in the swamps (your mileage may vary) to make more weapons if you're unhappy with your first choice.