Obtain a sword (not a katana) from the Cutlery Store (it's next to grocery store). You need to break the glass to access the action point. When the grocery psychopath comes at you with the cart, jump up on top of a shelf. Jump and press BLUE as you fly through the air. Because Frank is using the sword and not the katana, he will do a downward thrust with the sword instead of a kick. Swing a couple more times and jump back up. Repeat as needed and the boss will be dead in no time.