I have not defeated Bone Dragon myself yet but was given a formation to use.
Ezekial Adam Alci
Flavius Main x
x x x
Sub 1 Queen P
Sub 2 Khan or Maria

)I don't have Ezekial yet and have been trying with Herja)

You will need 6 Swords if you use Khan.
I know there are other threads on formations to use, but most of those were from before the earlier heroes got nerfed.
Good Luck