I beat that last boss 2 weeks ago with 27atk 675hp, and 30%dmgred. This setup gives you 3 turns to manage your board before going for the kill. Scrap upgrading heal stat.

General strat against bosses is to build the blue hearts to 10 in the first 2 turns while getting as much damage in. 3rd turn you finish it.

Tile swapper is very useful in getting hearts and attack tiles up through the triple/quad/penta tile interaction.