if you haven't already, build the stagbreaker to deal with the undead.

the reason for a lot of the spike in difficulty tho is due to the wetness debuff from the swamp, so because of that you should be prepared in a multitude of ways when getting ready to leave the forest ie defeat the elder.

you need your base to be completely surrounded by walls, far enough out so that it spawns the raids from trolls with distance from your main buildings, (you unlock troll raids after fighting the elder and they can destroy your base quickly,) and you need to be able to fight trolls with clubs.

you also need a working farm to get carrots to make deer stew plus later foods once you get turnips from the swamp.

you need the fermenters to make healing meads and poison resistance meads.

and on top of all of that your base needs a comfort rating of at least 8 in my opinion, because the rested benefit is what's going to determine whether you live or die in the swamp.

the longer you are rested, the longer your stamina bonus lasts and the less stamina it takes to do things like fight and run through the wet swamp.

and lastly, have a portal connecting to the swamp located in a treehouse on one of the indestructable trees.

once you have all of that you should feel a lot more confident in the swamp, iron armor isn't actually needed and the most you will need for bonemass is just the iron mace + troll armor and a good dodge skill.