When a Blunderbus is used against an engagement of henchmen etc. the rules read fine.

However, if it's used against a creatures with different armour values/defensive values that are engaged together, what do you do - use the toughest foe as the difficulty?

You use the Defense/difficulty of the target creature (whomever the attacker chooses at the target of the attack). Everyone gets hits for the same total damage, but that is reduced by their individual soak and toughness on a per-creature basis