I once went to the extreme effort of building a roof over the entire bonemass area so I could fight him while dry. Of course he spawned on TOP of the roof... :P

If there's a next time my advice is find 2-3 indestructible trees nearby and prep the ground/build your roof around them and just lure him over to the area after you spawn him. Dont forget at least 1 campfire to dry you out after you get back with bonemass.