Oof, I forgot how tough I made that fight. Most of your decks seemed good, it's just a tough fight that requires looking one turn ahead every turn. The AI is slightly exploitable as you saw when winning (he won't destroy two of his own stones to set up a good move), but there's also a specific strategy I use when playing that fight:
[https://rot13.no/?] Qba'g cynl n fgbar vs vg'yy trg qrfgeblrq vzzrqvngryl -- xrrc vg va unaq. Gura, svyy va n ovt ertvba nyy ng bapr, yrnivat ab tncf fb gung ur pna'g rnfvyl qrfgebl vg.?
Given how tough the 5 and 19 fights are, I'm thinking I might add a 3rd difficulty level of "Normal" difficulty, between the current ones.