Re: The Fairy, the Succubus, and the Abyss. V1B10P11 (Updated April 19th, 2014)
Bosses are all trial and error, I can give some hints (spoilers) for the bosses up to the white mage in the crypts, as I haven't really played since getting there (as I felt I was nearing the end)
I believe there is only one boss that doesn't kill you in one hit, and that's the black dragon in the garden area which instead did like 60-80% health damage (not hp, but the more difficult to recover health) And I actually liked that mechanic, it felt like I actually overcame a boss but took a beating in the fight and needed time to recover, which felt a lot more realistic than effectively save-scumming until you get it right ;]
There's also a few bosses that never really kill you, both of them found in the crypts and those I still count among the best bosses so far, simply due to the victory feeling more hard-won and less cheap.
But I don't dislike the boss mechanics on the 1-hit-KO bosses either, it's just that I wish it didn't just end the game right there. I'd much rather see something like a battered Sienna desperately escaping using the safety spell, but because of the circumstances she miscasts it and ends up some place away from her home and having to make her way back while severely injured (just put her in a randomized room with some enemies there, while her hp is drastically lowered to like 20% or something) This would also force some use out of the bribery mechanic, as you'll be in no condition to fight back against anything that attacks you on the way.
You'd still be able to save-scum if you really want to, but the trial and error of the bosses would be more integrated in the gameplay itself.