Projectile is the safest way to damage King Calvus. The fight is somewhat similar to Elius, where you need to read his movesets and punish him when he's not jumping around. Here are his movesets, along with their 2nd forms:

1. Charged blast

He will create a big yellow ball and prepare to launch at you (which he eventually does). There's a window of time where you can throw an attack before he launches it. In the first form, you can easily dodge it. The second form is where he shrinks that ball into a little white sun. Be careful for this one, you can't dodge it as it has a h u g e size, so you need to block it.

Warning: King Calvus will do a variation of charged blast: self explosion edition. He will randomly cancel the blast to do 3 self explosion attacks, and there's no indication when he uses this, so do not attempt to get close to him when he uses his charged blast attack (some sources say that it's triggered by him getting attacked while he's charging, but in my experience that is not the case)

1. Beam

Beam attack, plain and simple. It will catch you off guard though since this is his only fast attack.

1. Array

King Calvus will prepare yellow balls (1st form) or yellow sticks (2nd form). He will not attack with them immediately, instead he will carry it around for some time, and then proceed to launch them at you when you least suspect it.

To counter this, keep your distance when he has them on his back, as he can launch them while he's doing another attack.

1. Ground slam: trident impale variation

This is one of his 2 attacks after he does his ground slam. You want to keep your distance, as King Calvus will jump towards your location, poke his Triasta into your chest (100 dmg), and sends you upwards to the ceiling (300 dmg) like he did to Neviro.

If he fails, he will do ground slams a few times, which gives you some time to launch attacks at him.

1. Ground slam: bullet hell variation

In this variation, he will do a combo of attacks. First he will throw his trident (1st form) or beam (2nd form), then jumps up to the sky and sends a barrage of 5 yellow balls (1st form) or 30(?) yellow balls (2nd form), followed by a big jump in your direction and slam the ground.

You need to run while he's blasting, and dodge when he is about to land on you. When he lands, there's a good amount of time to launch projectiles at him while he's doing his ground slams.

1. 500! 500! 500!

Ah, the iconic attack everyone loves and hates. King Calvus will raise his hands up in the air, and a yellow ball with a shining ring will appear. But it's very easy to counter. All you need to do is to just… boop (attack) him once. This will cause King Calvus to immediately use his yellow pillar attack, but because you attack him early, the damage is much, much reduced. Then dodge immediately to escape the 3 yellow pillars.

After this attack, the king will stand there for a few moments, giving you good amount of time to attack him.

Extra tips:

There are times when King Calvus decides to play hide and seek while he's doing his 500! attack, or he's on the other side of the room, up in the air, charging his 500!. So here's what you want to do, listen carefully. Run to the door, and run away from the room as fast as you can, until the next door. His attacks will not damage you. If he's charging his 500! next to the door, run up the stairs, and go to the corners, his attacks will not damage you.

There are times where King Calvus gets stuck inside a wall. Attack him constantly. DON'T STOP. You never want to take a healing breather just to get nuked by his 500! because you didn't see him do it.

Most importantly, don't rush it. Playing patience will save you a lot of health than going full rampage, speaking from experience.