I know this isn't a terribly active topic at the moment, but I just wanted to chip in a thing that I didn't see mentioned in the thread here - I am pretty sure the reason that healing damages the final boss is due to an overflow. Normally when you cure something, it adds the current HP to the cure amount and sets the HP to that value, then reduces it to the target's max HP if it's over that amount. But the last boss has a huge amount of HP - so much so that if you heal it from its full health, the sum wraps the max integer (presumably 65535, the max of a 16-bit unsigned int), resulting in a total "less than" the boss's max, so it sets it to that sum - and then it checks the change between the old value and the new value, displaying that as a massive amount of damage.
This is why the cure spell healing the boss or damaging him seems inconsistent - after you've "healed" him the first time, his current health is much lower, and it can add your cure spell's amount to that without overflowing, resulting in normal healing.
Note that I haven't examined the game's code, or played it in a long time, so this is just informed speculation: According to
this FAQ, the Dark King has 40,000 HP. In theory, if you can wrangle a way to heal him by exactly 25,536 HP, you might be able to one-shot him with a Cure spell. I don't know how the phase progression figures into it, though (it might cap the damage at the threshold, or even have separate HP totals for each phase), so that might complicate the strategy.