Well, maybe is a little confusing and the font doesn't help.
Each turn, if you don't attack at all, the boss will get a x1.5 ATK. The more damage you make to the boss, the lower the multiplier will be.
You are supposed to attack as much as you can with the pirate, heal in every turn with the Sea Hag and risk for the biscuit with the fisherman (since has more DEF and an ability to receive less damage), but if you only do that, you won't be able to make it. You need to do some things before to be able to survive for 6 rounds.