Fleet Master Seahorn will fire his cannons at the nearest enemy he sees. Booty Bay Bruisers have gotten good at dodging errant cannonballs, mostly.
Send Flying Minis like Harpies or Whelp Eggs to kill Fleet Master Seahorn (Tauren with the Cannon) since Seahorn can't attack Flying Minis.
For every PvE mission, there is a campaign version and a Heroic version. When you first begin the game, you can only participate in campaign missions. Upon collecting 50 sigils, you will then unlock Heroic missions.
Campaign missions will grant rewards after the first successful clear with ANY family. Heroic missions must be completed with EACH of the 5 different families in order to receive the mission rewards.