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!