Use your minion - since you can't have friends port in to help, make sure you have a few minions in your deck just in case they get taken out right away. Have some healing treasure cards in your side deck, and ignore the fact that the puts a curse on when you blade. Take out his helper asap so it's just you and him, then keep chipping away at his health. You can't go for one big hit on this boss, it will take you a while.
