Melee doesn't work on this boss so you have to stick to fire magic.
His arms act as shields so he won't take damage unless they are out of the way.
You can remove his arms with enough damage but they have a lot of health so it's best to weave in and out of the bullets until he puts his arms up and then attempt to get 1 or 2 good hits.
He seems more likely to raise his arms if you aren't directly under him.

This boss does a ton of damage if he lands on you so avoid letting that happen.
When he lands he will send out waves of ice so avoid them as best you can.
He will occasionally leave piles of snow on the ground which get in your way and the way of your fire magic but they can be destroyed.
I found this fight easier without the idol since it seems to prefer homing in on the Snow piles.

Not much to say about this fight, it's another I prefer not to use the idol since it will home in on everything you don't want it to.
You can only damage him from shots to his mouth, you can't shoot through the head and tail of the worm.
At a certain point he will spawn a gene when his mouth opens and they can build up fast if you don't deal with them or kill him quick.

The boss shoots homing bullets from his eyes.
He lays eggs that eventually explode and send bullets everywhere.
You can kill the eggs with magic or by running into them and the boss also destroys them sometimes.
Again I found the idol made this fight harder so I wouldn't recommend it.
Lure him to the top of the room avoiding bullets and then get in some good shots, rinse and repeat until he goes down.

Not much to say about this boss.
Kill the bats while avoiding their bullets then when Zava becomes active kill her while avoiding bullets.
I honestly found this boss pretty easy.

Magic doesn't work so no reason to try.
I recommend using the cleria ring since you can't use magic
Dodge the bullets when he appears and run in to hit him avoiding the meteors as best you can.
If you don't hit him fast enough he unleashes an attack much harder to dodge so I found it worth it to take damage if needed to get in a hit.

I don't even know where to start with this guy...there is stuff everywhere, avoid it and try to hit him if you can?
On a more serious note, magic doesn't work here either so that makes things a bit tougher.
Goddess ring will occasionally freeze him which interrupts his attacks so go with that.
Meteors will spawn in the center in the room which will launch at you after a delay.
He will alternate between 2 attacks in phase 1, the first is a huge volley of bullets and the second is a smaller volley of bullets.
When the goddess ring activates it will interrupt the attack so take that into account when preparing for which volley to avoid next.
In phase 2 the meteors continue but the volley of blue bullets is always small
He has 2 new attacks, the first one is 3 lines of yellow bullets which rotates and is fairly easy to avoid if you have enough room.
He seems to like to move close to you before using this attack so try not to get pinned in a corner.
The second new attack is a blue beam which he starts shooting and then can move.
Dodge all the stuff and get in a hit or 2 when you can and then enjoy the well deserved ending and achievement!