What worked for me was entering the swamp only during day time and wandering around the edge of the swamp and stocking up on blood bags from leeches and entrails from dragur until I had a nice supply of medium healing meads and sausages. Once I had better food I found the crypt that was closest to the safer adjacent area (meadow or black forest) and built a small base in that area to house a portal back to my main base where my rested buff could be maxed out with the decorations I had built.

Then go back to the entrance of the crypt and build a work bench outside to keep stuff from spawning outside the crypt while you explore. Once you go into the crypt there are 3 possible paths from the entrance (just like the dungeons in the black forest). To start, ignore any that are blocked by scrap piles, none of the swamp critters can go past the scrap piles so you are safe from those paths. When you have identified the unsafe path explore it first. I've noticed that the body piles spawn mobs that are tougher if it is night outside and they seem to spawn them faster so explore the crypt during the day also. After you have cleared any areas that are not blocked by scrap plies find your scrap pile that is blocking the path and start on it. Work on one side of the scrap pile only until you have opened enough space to see into the next chamber but not enough for you or any monsters to get through, then either use a bronze atgeir or your bow to clear out any monsters and body piles in the next chamber. Once the chamber is clear finish off the scrap pile.

Repeat until you have enough scrap iron to make an iron buckler. An iron buckler is strong enough to stagger anything that you will run into in the swamps. This will put you back on par with the creatures in the swamp and you can continue playing as you have in the black forest. Clear our enough crypts to get fully upgraded iron gear and prepare for your fight with Bonemass.