Your talking about the duck of death correct. Who is immune to all attacks until you click on one of the fountains and bless your silver bullets or swords for that matter . Once you use the fountain all your companions should have a halo above their heads and be able to damage the duck. Then its just a matter of hitting him and his minions. Then you fight him again on his ship only this time you dont have to do anything but defeat him and 2 minions not too hard. Focus on the minions first once they are dead then kill english bill another stratgey in all battles most use is to kill all the gun fighters or bow users then move to the swordsmen or staff heavy weapon users.. The reason for this is that riposte does not work on ranged attackers only in close combat against swordsmen and the like. So while you are hitting the ranged attackers any close combat attackers usually suffer some riposte damage or vengeance strikes during the computers turn which saves you time and damages them. This maximizes your damage output every turn. Hope this helps you strategize better.