Being "very resistant" to fire and pierce damage means arrows is definitely not the best approach.

https://valheim.fandom.com/wiki/Bonemass

As already stated, mace works like a charm. I charge right up to the "sparkles" as he spawns and can get him down 30-40% before he begins moving. At which point it become a game of "ring around the skull." Charge - use meads - recover (and wait for mead cooldown timers) - repeat.