I have no idea why this challenge, unlike the previous one, doesn't have a fixed difficulty. It seems silly to me, and I'm very unlikely to lower the difficulty to go for a sub-20 win (I find playing in low difficulties rather boring).

So, this was my best run on HK difficulty:

Also, fun fact, they forgot to deactivate the "The stars my destination" achievement for this scenario, so it's probably the easiest and fastest way to get it. Even in this 26 turns run I got every possible star.

Forgot to upload the save file: Franks Turn 26.ctr