The victory conditions are to defeat all enemies, but you don't have to do that, and can't. The only enemies you can defeat are the three witches. What you really need to do to proceed with this level is to get hit by the canon six times. That can mean one character gets hit six times, two characters get hit thrice, etc. It will deal 15% Morale damage per hit. Once that happens, the story should proceed. You shouldn't waste too many cards in this part, because they do not refresh in 16-3 BUT WAIT. The first of the two Duds available in this level appears in this part; you may wish to grab it now so you don't forget it later. For your information, you need seven Duds over the course of the next 23 battlefields to get the strongest item in the game, as well as three other neat items, so it would behoove you to nab all of the Duds.