Bonemass isn't scary he's a giant blob, as a prank someone summoned him on me and I literally just ignored the boss and returned when it was time to take it out. He's background scenery unless you decide to confront him.

Also mountains want you to learn patience the biome punishes those who rush by draining their stamina and finishing them off with fast moving enemies. Stealth allows you to avoid the majority of them since you will see them before they see you.

Bonemass punishes passive behaviour since the biome's poison and ranged attacks encourage you to take out the problem rather than sitting back with a bow or something.