Ok, I decided to unlock a deck (100 teeth) and try a different approach. This time, I always drew new cards (25 teeth) if the one I had didn't match the opponent. Half of the times, I got a good match, other times nothing better. I only used the free special powers. I always healed my card (10 points = 85 teeth). I didn't beat the boss, but I got 4 gold, some silver and lots of bronze keys. In total I spent 800 teeth, almost 1/3 of my stash.
So it seems that trying to beat the final boss is impossible and/or not worth it.
Next time, I'll try redrawing, not healing the cards and buying more special powers.
This time, the free diamonds will
actually be used for testing