It should be said that the bosses are tougher the more players are in the game, at least I believe this is the case.

I helped out a friend vs the Elder some while back, and he wasn't exactly carrying is own weight. My gear wasn't the best either, but the point is that soloing a boss that is buffed for two or more people is a lot tougher than soloing him when you're single. I think it's the case anyway. Bonemass can be very tough as it is, but it may be something to consider if you or your friend aren't very good at the game.

One way to solve the issue is to mod or just plain cheat your way through. As long as you're having fun.