For a guide to finding the door to this miniboss, please read Sin Ogaris's walkthrough or watch xGothic93x's video playthrough. Both are great. This guide is for the battle itself.
This miniboss is the hardest of the four 'upgrade the ability' minibosses, so I would recommend doing the other ones first. The other three have a small moveset each. This miniboss uses a combination of all three of the other minibosses' moves, increased in difficulty.
Like the other three, it has a set base moveset that increases in difficulty as the battle progresses.
The trick to beating this miniboss is patience, hit-and-run tactics, and dodging.
Melee moves:
Bonk: This is an overhead slam of its weapon into the ground in front of it. If you are far enough away, it will not launch a follow-up attack. You can dodge away from it while it attacks and then roll in close to attack it back. Be careful not to be too close, or it will do the next attack.
Bonk-sweep/Bonk-lunge. This is an overhead slam of the weapon into the ground, followed up by a second attack. If you are near its front, it will lunge, if you are further around behind it, it will sweep. Dodge the first attack, then hold back and dodge the second. It will pause briefly after the second attack and leave an opening for a few slashes.
Later on in the battle, it will combine a melee attack with throwing a bomb at you. The visual clue for this is that its head will disappear briefly while it launches the fireball.
Fake-Out: This miniboss has stolen the Hookshot move from the Hookshot miniboss. Rarely, it will Hookshot to one of the points, and instead of using its bomb barrage, it will turn back around and jump back, using an overhead slam attack towards you. If you are close to it, it will follow with a sweep/lunge attack and later with a bomb as well.
Jump: If you are standing too far away, sometimes it will run at you, and you can end up just running rings around the arena. Sometimes it will jump towards you and slam its weapon down onto you. This has mainly triggered for me when I try and use a ranged attack while it is still in the arena with me rather than on one of the three platforms. You can dodge it more easily if you aren't charging an attack.
Ranged Attack:
Stun: It will slam its weapon into the ground and 'stun' you. A symbol will appear underneath you. Your movement speed will be drastically reduced, and you will not be able to dodge or melee attack. Spam the dodge or attack buttons to break the stun. This is a concentration move, so for several seconds after using it, it will be stood still, holding its weapon in the air. You can use this as an opening to launch a bomb at it if you want. The miniboss will not remain in the hold position for too long, though, so you won't have time for many attacks, and it will start moving again as soon as you break out.
Bomb: It will launch a bomb at you. Its head will disappear into its body whenever it is about to use fire magic, so you will know if it is about to do it. There will be a small symbol on the floor telling you where the bomb will land so you can dodge it.
Bomb Barrage: It will Hookshot to one of the three points around the arena (damaging you if you are in the way of it/its fireball) and launch a barrage of bombs into the arena. The bombs will aim at small symbols on the floor. There are a few seconds before the first bomb lands, so you can use this opportunity to launch a ranged attack at the miniboss. I would recommend the fire spell as it does more damage than the bow and takes less time to charge than the bomb. Look out for the fake-out. If you are charging a ranged attack, it is hard to dodge in time if it jumps back at you.
Unlike the Hookshot miniboss, when this one jumps back, it will land roughly on the ring of stones closest to the platform rather than the centre of the arena. When it jumps back, it will sometimes launch into a combo of Bomb+Melee attack.
Defence:
Fire Shield: It will summon a fireball that will circle it and damage you if you touch it. It will summon up to three at a time. (when I beat it, it only had 2, but another time I fought it, it had three) They follow the miniboss around the arena, including when it Hookshots away, making it harder to dodge.