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.
Finnaly finish he game this boss are the last one not more capter. The only what remain now are the events.
It makes me or with each new event they reduce a day; and they ask for more and more things as an objective. The first was 5 days; then 4 and now with this last only 3 days to complete it.