Ugh, Stupid Phantom.
Yea, the last few bosses auto heal every few turns (either 4 or 5 depending on the boss and it's always a few grand of HP).
What demons are you using?
Level doesn't seem *as* important as gear and skills. There is a skill called Shadow Assault....I'm pretty sure that's what it's called. I found it and put that on my samurai. This gives that character a chance to auto hide at the start of battle, then you can use hide during battle (it's like Concentration). Each turn the character is hidden, their damage goes up 1.5, 2, 3, 4, 5 times the base damage. It maxes at 5 and ignores enemy defense (cited from the google doc). That skills is super useful for damage, and could do 4 thousand to even the last boss.
I had only found Mars Bane and gave that to my Samurai (before I had found Shadow Assault). I used Cyclone on my Fighter (you may be able to atleast find Rising Hit (another Fighter skill) which hits several times). True Snipe is probably the skill you're thinking of, that drops an enemy's evasion and always triggers Pierce. But it's only usable with bows maybe? Couldn't say for sure.
Also, Berserking Mars helps a ton if you aren't using Chronus for defense. The Bosses that heal you just need to be able to store up several of these larger attacks, so make sure your hit is buffed up so you don't wiff. To help with fast buffing, I found Fast Cast and gave that to my Healer.
All this aside, having some skills for hitting multiple times, like Shadow Assault, will help a ton, and having Mars Berserked will also help as she does great damage when she finally decides to hit.
And I played the game through on Hot.
Oh, final note, Whistle can be used preemptively. Had no idea for nearly the entire game. If you cast it, it throws it up on the screen, top right, like the Force buffs. It, like Force Guard (prevents the enemy from wiping your buffs), functions as a single prevention of those enemies who change your line order or remove your buffs. Once Force Guard or Whistle is "consumed" you can cast it again next turn and you'll be ready the next time the enemy tries it again.
Hope this helps and wasn't jibberish!