We finally completed our base portal hub and storage section, and I relented on refusing to kill Bonemass. The question was how.
Now my son is playing a variant where he only uses a bow the entire game. We had obtained 2 Finewood bows before we killed Eikthyr, and had not upgraded them while we were running the swamp.
So we built this sniper tower above the Bonemass spawn point, with a direct portal to it. And we went over with around 400 wood arrows each.
The first summon, Bonemass wandered off north of us and did not come back. This sort of messed up our plan. Plan 2.0 involved raising earth walls around the Bonemass spawn, then luring back in, and building an earth wall to complete the trap. Much as you would do a 2-star boar. Only with earth. And a lot of nasty poison. And blobs.
And so I valiantly held guard at the top of our sniper stand while my son who is in the full bloom of twitch play ran off and fetched Bonemass. And he trapped him. And he made it back alive to the stand.
We started peppering Bonemass with wood arrows. I think I had 24 skill, and my son had 34 to start. I was hitting for 8-11 damage a pop. Bonemass had a ton of arrows sticking out of him. We went through the day, and night came. My son said 'lets rest' so we ported back to base and slept.
Upon return, Bonemass had reset his life to full. Oops.
So someone had to be hitting him at all times. We were going to shoot him nonstop until it died.
Then we found a few more problems. We were running low on arrows. And our bows needed repairing. So we would rotate one out while the other kept up the fire. It still was slow. So we made 2 extra bows and I carted out 1000 arrows and put it in a chest. Now the supply problem was solved.
But the stamina problem was bad. It was raining all the time, and we were wet, so I built a roof during the fight. We dried out. Stamina came back faster.
And finally after about 2 day/night cycles of incessant fire, we had him.
It turns out though you still level bow faster by shooting it into the floor or something like a rock. It is xp per click, not xp per charge, and the time spent to charge, even with the bonus is not better than rapid firing arrows into the floor as fast as you can.
I suppose we could have rapid fired arrows into Bonemass, but he moves around a lot, and being cold etc hurts your stamina. Maybe with a proper rest dojo in the the tree, and an even more restrictive earth wall around Bonemass, and not caring about whether you kill him or not, we could perhaps farm up bow skill faster.
In the end it took my son 10hrs of firing arrows into the floor, and over 30 chests (24 slot) of wood turned into arrows to hit 100. He says a single tap of the left click does the damage of a full charge now.