All the bosses can be beaten solo - yes they are tough but honestly who wants an easy boss (at your level) if you stuck on Bonemass go back and kick the Stags head in a couple of times to practice should be easy at iron level.

You will get over the Bonemass hump and eventually he'll be as easy as ol's Stagfeatures is to you now.

Check what the bosses are vulnerable to and use those weapons. Anything can be a shot to death as Brock said above with a good vantage point and enough arrows. But keep at it and maybe you'll find a way that nobody else has or a combi of methods. Luck to you Viking!