Seems the OP forgot there are more spawnpoints for bosses as just one.

So instead of taking a fight with Bonemass in a swamp which is surrounded by plains, it may have been easier to look for another spawn point with better access.

I'm aware that may have taken more time - or less if the online map generator is being used - but it would have given the OP a better chance to clear the swamp, prepare for the fight and do the offering of the bones to have an easier fight.
And later being able to come back to do the entire fight again with Plains level gear at the first spot.

The only thing that can't be done is using vesigvirs in sunken crypts, as they would point to the first found spot. But finding a second or third location and having it added to your map is easy enough, as there's always a rune stone near the altar. And "reading" it will add the boss location at your map. No matter how many others you've found before.

Thorin :)