The dungeon is shaped like a heart, and the secret area is in the one square on the map that's missing to complete that shape (or if by "secret boss" you already mean the one in that room, then unfortunately there isn't anything else)
Although, if you ask me, the Fierce Aunnlettes aren't worth beating. The fight at the end of the secret area is the worst thing ever, even with the highest possible stats. Whoops!