After banging my head against the wall, I managed to beat it
Last boss I single stacked Paladins with the Crit to Demon hero. Stoneskin reduces losses to no more than 2 per "dancing blade" spell. I spent the first half of the fight running around building up my kills (I had the ability that gave me +2 attack for every kill). Since I single stacked, I could use level 2 Demon Slayer @ 10 mana instead of level 3 @ 25 and that would 1 shot every stack unless it was the Tier 5 Demons. Even then, I would 1 shot them if I crit. Also, since it was only 10 mana, I could double cast. Everytime 2-3 turns, I'd Phantom myself to resurrect my lost Paladins.
After I built my Attack to 150+, I just beelined for the main demon.
Turn 1 = Cast Phantom, Attack with main, Second Wind from phantom to main, Second wind from main to Phantom, Attack with main, Attack with Phantom.
Turn 2 = Attack with main, Fit of Energy, Attack with main, attack with Phantom
Turn 3 = Attack with main, Fit of Energy, Attack with main, attack with Phantom.
Repeat Turn 1.
I was critting for over 14,000 damage on the boss after I built up my ATK and took him down after 5 or so turns of serious damage (interrupted my spawns of course)