well... most of the bosses are easy. that's just how the game is.
you can kill eikthyr naked. you only need to learn to dodge his attack
same thing with elder (or you can cheese him by hiding behind pillars and shooting arrows)
the only challenging part of bonemass is it's poison cloud. which is hard to avoid if you're melee player. but as a ranged you can just build a tree house and there will be nothing it can do.
same with moder. everything is easy to avoid. and you can kill her fairly easy even as melee. it'll just take a bit longer then a bow. so the only challenge will be from other monsters that may visit your fight (especially golems)
not sure about yagluth though. the boss itself is easy. and i've killed him a couple of times solo... but i've read that his hp regen scales with the number of players. so it might be a problem in a big group. (as a tip to make fight easiers: lure him in a position so that there will be obstacle between his back and meteors he summon (he always summon them behind him). this will negate a lot of his damage. everyhting else is eikthyr 2.0)