Additional "minions" or "bad guys" don't spawn because of the players' death... nearby baddies may be drawn into the battle as the player runs around a wide area trying to avoid Bonemass.

Bonemass is a (seemingly) tougher boss due to the location, which is the swamps... water / wet weather and cold conditions lowers a players' regeneration rates, as a general mechanic in the game, so the swamps are particularly challenging, but the player can compensate for that by equipping poison resist mead and stamina regeneration mead, to use as needed during the battle... (and have some spare ones at home in case of death)

In order to help even the odds, during the Bonemass battle, It helps to do a bit of terrain prep before the battle takes place. It is much easier when the player raises all the land around the summoning altar, to get rid of all puddles of water, (where leeches can attack player), so the entire battle can be done on solid ground... You DON'T want to have any puddles in the battle area.

It is best to go around and clear out all enemies in a wide area to ensure extra mobs aren't drawn into the battle, once it starts,

Bonemass is resistant to almost all forms of damage except it is WEAK against blunt damage (MACE) (as are almost all mobs in the swamps). Mace is the best weapon to use in the swamps especially against Bonemass. (Shooting Bonemass with arrows is probably the WORST way to try to fight it.)

Bonemass will "telegraph" it's attacks, so the player usually has time to back away when they see the 'wind-up' happening, and in-between those times is when the player should run in and attack with the mace.

Have a portal set up nearby so if the player dies, it is simply a matter of eating foods to get health bonuses back up, resting by the fire to get rest bonus back, then use portal to go get his / her gear back.