If you want to defeat the demon dog boss you must first change your fighting style to the eagle with it you can kill lots of weak enemies with one or two hits then go and help the witch by killing the minion green dogs first before she became unconscious, she will be healing you after that. then change your fighting style to the wolf and attack the red big dog until it dies. if you become low on health try to escape and kill the minion dogs first until you get more health from the witch then try to finish the boss.