Ys Oath in Felghana Nightmare mode playthrough part 10 : Istersiva, the Corrupted Crystal Mutant

The crystal is already corrupted, and THEN it mutated, that's the result.

I have to admit I kinda like this boss battle, there's plenty of action. It also got MUCH harder than in easier difficulty level.

First half of the battle is pretty straightforward. He'll come out of the ground and do two things. If he stands there and do nothing, he'll wait for you to come close and then try to smash you. This is usually easy to dodge, but as you can see, the shockwave tends to stick around longer than it should. If he doesn't do that, he'll spit some crystals. The crystals can become extremely dangerous if you let them be. If there are 3 or 4 they'll start reflecting the laser beams all over the place. You can see it a bit at the end of the battle against the worm, but it can get almost impossible to dodge if there are 5 or 6 crystals out there. The easiest way to beat him is to use wind magic in front of him to try and destroy the crystals as soon as he spit them. Try to avoid getting behind him, or he'll use a spinning attack that's really hard to dodge.

Once he's done, stay clear of the remaining crystals, since they explode and deal damage.

The second part gets pretty fun since he has a fair amount of attacks. Sometimes he'll just move through the stage leaving eggs behind him. Those eggs explode and deal around 60 damage. You can also attack him from behind if you're crazy, but it's really not worth it since you can't deal a lot of damage that way. He'll also shot 4, 6 or 8 sparks that will explode and spread, those are easy to dodge, most of the time, but they hurt a lot, over 60 damage again. Then he'll start spamming his beams things all over the place, but there's a pattern and it's not too hard to deal with. Finally, he'll sometimes create some weird shield around himself and charge at you. The hit window of that attack is pretty small, so you shouldn't be hit by it. It deals over 120 damage though, so that's more than half your HP.

Where it gets really hard is when he starts chaining his attacks very quickly and for a long time without giving you a chance to attack. Stay concentrated, try not to overuse your wind magic, since if you run out of MP and you need it, you'll get hurt.

When he opens up, close in quickly to avoid the pink beams, and try to time your jump and the wind magic so you can avoid the explosions from the eggs that he spawned.