Ys Oath in Felghana Nightmare mode playthrough part 21 : Zirduros the Steel Beast Cannon

This boss is easy, he's mostly just filler between this area and the next one.

The best way to damage this bastard is with your earth magic. You can only hit him from behind, and missed regular sword swipes will knock you back, which is annoying. So just charge up your earth magic, and unleash it wisely, he'll go down quickly.
Anyway, he'll always do the same things. He'll shoot a wave of missiles, they cannot hit you directly, just the explosion will damage you. This means simply a well timed jump will get you out of it.

The homing missiles are more annoying. He'll shoot three pairs of them, and they'll follow you until they explode. The explosion stick around for a few seconds, so be careful. A direct hit obviously also hurts, but this can be avoided fairly easily just by walking around and jumping.

He'll also shoot everywhere with his gun. He'll altern between short and long range with this attack too, so watch out. Earth magic is the best way to avoid it and deal damage at the same time against this attack.

Sometimes he'll also try to jump on you. Well, not quite, since what will deal damage is the shockwave. You can dodge it just by timing your jump properly, but Earth magic is way safer.
He'll also sometimes shoot you with a huge laser beam. Uh... I'd love to tell you that it can be dodged, but not really. Unless you somehow manage to predict it you might be able to get some earth magic to block it, but otherwise, you'll get fried. It doesn't hurt that much though.

Finally, he'll occasionally throw landmines behind him, they're nothing more than an annoyance.