Ys 2 Chronicles+ Gelaldy (Nightmare difficulty)
7Gelaldy is a tough cookie. He is only vulnerable when his mouth is open, and even then, the head/tail of the... worm(?) thingie he releases can block the fireballs. Once its health gets low enough, he releases small monsters as well when he opens his mouth, which take no damage from fire and can therefore block the fireballs aimed at the main boss as well. Quite a pain.
Gelaldy will try to follow you. Sometimes, he will make a charge, and if you get cornered against a wall from it, you're dead. After some time, he releases that big worm thing. He can actually be damaged while he's releasing it, but it's very precise, and you won't have time to do that much damage. It takes a lot of practice to get to learn a proper angle to shoot him from to be capable of damaging him like that (I'm not exactly mastering it either, but I do manage to hit sometimes, as shown in this video). The snake will try to coil around you. If you're caught inside, you're dead, period. So never let the worm circle around you.
After some time, Gelaldy will open his mouth again to let the worm back in. That is the time where most of your damage will get through. Ideally, you want to make it so by the time Gelaldy opens his mouth, the worm's "head" is as far from Gelaldy as possible, because then he, logically, keeps his mouth open longer because it takes longer for the worm to get back in.
At some point, Gelaldy starts releasing extra monsters. As mentioned in the initial paragraph, they are fire based regular monsters that absorb (block) fire, so if they get in the way of the mouth, they can prevent damage on the boss entirely for one "round" of Gelaldy having its mouth open. I prefer to try to kill them if Gelaldy and the worm aren't completely in the way. Otherwise, just try to avoid them. The battle can get rather erratic when there's two monsters running around, the worm, and Gelaldy himself.
The Hawk Idol is very important in this battle to hit him with "curved" shots.