Finally beat the prologue boss on hard... that only took like... 2 hours.
I really don't think there's a 'good' way to win that. If the boss decides to keep healing dishing out 1-2k damage per turn is simply unreasonable without getting destroyed. The only particularly useful tactic I noticed is if you space your characters kinda far apart (but not too far apart) then the boss will waste some turns going back and forth. And you have to just kind of get lucky on the AI being dumb and every once in a while you actually get to attack.
Even on my winning fight the boss healed himself about 6 times which meant he had 15000hp. It was like 20-30 minutes of heal, heal heal, poke for 400 damage, heal, heal, heal, heal, poke for 300 damage.
Has anyone taken him down on nightmare? I don't want to even think about that.