Archvale

The final boss of the game.

After you defeat the 7 bosses around the map to acquire all of the Arch pieces, head to the Twisted Caverns. You should see a green eye looking thing on the map in that area if you've explored it fully.

Head there and build the gateway. Prepare for the 3 stage final boss fight immediately afterword. It's not really that hard if you're fully upgraded. You could probably tank most of the damage and just keep firing/slicing and heal every once in a while.

The three most powerful weapons in the game, one from each category are:
Magic - Arch Lexicon. Obtained by forging it in town using the 3 other high-level codex's found around the map. See the Well Read achievement for details.
Melee - Avarn's Sword (there are a couple melee weapons with higher dmg but they have lower range and rate-of-fire). Obtained after defeating the Banished Executioner in the Molten Rift.
Ranged - Terracotta Bow (my favorite and with the right Badges it outputs crazy dmg). Obtained from an NPC in a random part of the Jungle area.

Credits are unskippable but not that long. You can continue playing after you beat the game so don't worry about missing anything.

This is not an endurance fight, you have to have superb damage output as the boss does incredible amount of damage. (And this is an understatement.)

Be prepared to have several tries as the 1st phase is mostly about RNG, I rarely made through the falling red-black bolts and sometimes had them come twice in a row. Try to be quick and do a lot of damage in just seconds. Health potions I used during start of 2nd phase - after the first round of 2nd phase (when the boss blows three green bolt circles) and then whenever I could, in 3rd phase I used a few when I had a space in between the bolts. So try to just blast through the phases, it will be a quick run if it goes well.

I tried several different badge builds only to be wiped out in phase three of the boss fight. Here's what I had equipped when I finally finished the King:

Badges (usage in *)
** Sapphire Brooch - Increased magic damage
* Star Magnet - Massively increased pickup range for magic shards
* Blue Lotus - Enemies drop more magic shards when damaged or killed
* Ultra Shards - Increases the strength and duration of magic shard boosts
* Adrenaline Rush - Attack faster after taking damage
* Fairy Fighter - Summons a fairy that reduces enemy defense (note: in the 3rd phase this was on a side monster, didn't matter though as I got the kill anyways)
** Dark Heart - Defeated enemies provide minor healing

Also my equipment:
Weapon: Archvale Lexicon +5 (upgraded to max)
Head Armour: Phloem Hood (Magic DMG +15%)
Body Armour: Phloem Chainmail (Magic DMG +15%)
Sapphire Ring +4 (Magic DMG +20%)

Had also collected all hearts and healing potion upgrades (achievements unlocked).

Good luck!

After acquiring all pieces of the Arch head to Twisted Caverns. There will be a green eye icon on the map if you have already explored this area fully. If you have not, you will have to continue exploring until you find a wall with eyeballs all over it. Simply click on the wall made of eyeballs to enter the final area.

The final boss has 3 stages. I found the first stage to be the most annoying.

Truthfully, I did not bother using any of the most powerful weapons in the game. I stuck with my favorite weapon I was most comfortable with.

My gear set up was as follows and I only needed to use 2 or 3 health potions during the battle. Keep in mind I had already found and purchased all heart containers and acquired all defensive boosts from shrines.

Equipment:
Doomerang +5
Hyperium Mask
Zephyr Breastplate
Deathcap Ring (adds +10 defense and poisons all enemies when you take damage, was a random drop)

Badges:
Emerald Brooch
Arrow Bender
Echoed Blasts
Golden Heart
Adrenaline Rush
Fairy Fighter

I was going for ranged damage.
Doomerang is craftable at the anvil from I believe Bloomerang which is also something you need to craft. It is by no means hard to get the materials to craft these. They are towards the bottom of the crafting list at the anvil.

I had zero issues beating this boss on my first try. Hope this helps anyone struggling.