Page 36, section Mob Attack wrote:
When a Mob attacks use the Attack dice indicated on the Enemy
card to resolve Combat rolls, no matter the number of Minions
in it. If the Boss is carrying a Weapon that matches the Attack
type being performed by the Mob, its dice are added to the Mob’s
Attack. The entire Mob focuses its Attack on a single target.
(FWIW, IMO the wording throughout the book is unclear on that subject up until page 36, where it's made explicit. Earlier in the book there is a mention of the Boss benefiting from it, which implies that the rest of the mob doesn't.)