Bonemass is a massive pain to fight and it took me like 5 attempts to fight until I was able to come up with this method. Bonemass will take take pretty much no damage from everything and even using blunt damage (which is the only damage type he is weak to apparently) , he will still take very little damage from your attacks.
How to Make The Bonemass Fight Tolerable Without Cheesing
If you haven’t already, go get yourself some basic iron equipment (a bow, armour maybe a shield + melee weapon. Then get yourself some higher tier foods ready like queens jam and sausages.
You will need to start making a few meads, make some poison resistance meads (one batch should be more than enough), and some frost resistance meads (this will be used later in the guide trust me).
Optionally to make the fight easier, make yourself some minor health potions and tasty meads.
This is where the trick comes in, this will sound odd at first but you will want to go into the mountain biome, its actually not all that dangerous as long as you have some okay armour. What you want to look for is some black looking rocks that can be found on top and around the mountains called “obsidian” and try and hunt some enemies called “drakes”, you will need to hunt them to get their “Freeze gland” drop. collect at least 8 freeze glands 32 obsidian (that’s how much I got and I had around 20 arrows left at the fight). Now head back to your base and craft these items to make frost arrows. these arrows are very powerful and will take good chunks out of bonemasses health.
You pretty much have everything you need, if you want the fight to go smoother, go build a portal near bonemass so you can quickly teleport back if you die. It is helpful to upgrade your bow too and if you need extra metal, you can break some of you forge upgrades if you are desperate. wait until your food buffs expire because your food buffs give the strongest boosts right after you just ate them, use honey as it gives the highest health regen in the game. Also drink you poison resistance mead too and head off to fight bonemass.
Try not to get too close to bonemass, just fire fully charged frost arrows at him over and over again and try to avoid the enemies he spawns, quickly fight off any enemies if you get surrounded. All of bonemasses attacks have poor range and can be easily avoided. Use the minor health mead if your health gets low and use the tasty mead if you want a quick boost in stamina.