To defeat the Old Bones Gang you need to complete the Skull Cannon and get it loaded...
To load it you'll need six skull cannonballs and five machinist's gloves.
The Skull Cannonballs are crafted in the Cauldron on your homestead:
And the Machinist's Gloves are crafted in the Ghost Town's Clock Tower:
Finishing the Gang off get's you a gold skeleton key!