The best swamp weapon to invest in is the mace. All other weapons are a waste of iron, don't waste your iron and focus on anti boss gear to get the next tier.

Bonemass is largely about trusting the speed of your standard power walk. Sprinting around will burn far too much stamina and you'll run out when it counts. Walk in, whack whack whack, walk out. It's scary but it works.

Use the Hoe to flatten the arena. Removing water is hugely important to eliminate the automatic weapon unequip that happens in waist high water, and it also removes leeches from the battle.

Of course, use the highest quality food (turnips, sausages, etc.) and poison resistance mead is pretty much mandatory. Bonus points for having a few healing potions and stamina restoring stuff.

It's a good idea to have a small portal base somewhere near the boss arena. Things happen, and you might want to fight the boss more than once for reasons, so a portal is nice to have.