i have 2 questions
1. is there an S rank or any rank above A in adventure mode? nvm i got S rank, but i guess the new question is if there's anything above S rank
2. will there be a way to play the boss fight in free play?
1. There might be a rank above S. But you'll have to be a perfect and pretty fast minesweeper player if you want to achieve it.
2. There may be a way to fight the boss in free play in the future. (But for now, there is a super-secret keyboard shortcut that you can input on the map screen to get to the boss right away. Just press and hold a certain key on your keyboard for a certain number of seconds.)