Ys 2 Chronicles+ Druegar (Nightmare difficulty)
2Druegar isn't tough per se, but if you mess up, you can pay dearly for it. You need to throw fireballs at the very center of its body, from underneath. As far as I can tell, it doesn't work from above. The main strategy is therefore to shoot fireballs to hit the body, lure it as far up on the screen as you can, hit him with fireballs again, and so on.
Every couple of seconds, he shoots six or so homing projectiles. By dodging to the side, they should hit the wall and be destroyed. If they don't touch a wall (or Adol), they home in once again. Druegar lays a bunch of eggs every once in a while, which explode into a lot of projectiles after a while. Those don't home in, fortunately.
The eggs can be destroyed simply by running into them, or hitting them with a fireball. I tend to try to break a few to have some breathing room when they explode. Druegar's legs can also break them apart.
The end of this battle was kinda poor, and I took several hits in a row before finishing Druegar off. Still, that was my best run up to that point (seeing as I started recording each attempt from the get go) ; my first victory against Druegar. Although I wish I could have showed off a better performance.