How do I win the battle without the dragon knight being petrified?
I admit this game gives too few hints (basically none at all) for this, especially now that some clues are given to defeat the final boss...
- Get Dragon Knight in your party. The adventurer's class before the transformation doesn't matter.
- Equip him with Sword IV and Might Belt to maximize his Attack stat. Also give someone a Hex Hat for debuff spells.
- Enter the final battle, and damage Warlock until he has barely more than 50% HP (below 55% should be enough even at the lowest possible adventurer level). Alternatively, you can just set his HP to 50% (1st phase) with the post-ending unlock.
- Boost Dragon Knight's effective damage with Empower (or Inhale), Corrode and Shrink. You might also want aggro-control skills to keep Warlock's ATK debuff from Dragon Knight.
- Once everything is set up, his Glorious Finisher should do enough damage to skip the second half of the battle.