Finally got board clear with 13 hearts remaining, which I think is the best possible run for board clear. To do this, you need to all the free XP before using any heart scrolls (egg+chests+defused mines+gnome). You use the first scroll at 11 health.
Need good RNG in regards to getting egg/treasure chest and quickly locating slime wizard/rat king. Basically need to have as much board info as possible ASAP to remove empty squares in order to get the gnome (this also requires careful analysis of squares).
Took me about 30-40 attempts, restarting when initial RNG didn't look promising. Had about 5 serious runs before finally getting it, having to blind guess between mine or gnome (25-50% success chance) and failing until the last attempt where because of good enough RNG, didn't have to make any blind guesses.
Probably took around 4 hrs total, went slowly to avoid any mistakes and was clenching whenever I had to make completely random guesses. Fun challenge, though can't imagine trying to speedrun it, would be so stressful.