Use Needle or Obsidian Arrows not Poison or Fire ones. Poison Arrows DoT effect doesn't stack so you want high pierce damage ones instead.

If you got silver armor and weapons you can also tank during the Bonemass buff whenever she lands. With the buff active she doesn't deal much damage. Keep in mind it will be a long fight. I did it together with a friend and we still went through the entire Bonemass buff.