ball spawns in their direction. After it spawns the trick is to pass it back and forth between the ranged group and the melee group. On the 5th hit the ball must hit the boss or the damage the boss is duking out will start to become unbearable. As big as the bos sis he has an annoyingly small hitbox. To ensure that the ball hits the boss and not you, your pet, your last lingering sneeze molecule, melee run through the boss and keep attacking on the opposite side at max range. If you’re still having problems with the ball hitting someone, have melee run through and keep running, only turning around once the boss has connected with the ball.
If you get the debuff Blinding Shadows, make sure you’re not near the ball. It won’t affect the back and forth, but it could kill you if the debuff blows up the same time the ball hits you. A nicely (read: lucky) timed Stoneclaw shield might save you.
Also, don't stand in front of the boss. Bad things happen.
Loot: Tier Gloves, and the usual ring/trinket suspects.