It's really not that difficult solo. I beat Bonemass I believe 40 hours in?, just beat Modder cause I had everything maxed and wanted to move forward. (70hrs~?)

These bosses are also easy to cheese tbh. Bonemass, build a treehouse and stay there with poison potions. Make 200 frost arrows, stay behind a tree that fell down, he'll just stay there.

Next boss was even easier. Dying on a boat over on the next island is hell in the beginning I agree (So is the boat in general lol). But once you'll beat those moments, you'll feel even better about yourself.