The final boss of the Fight Kiln, he actually doesn’t pose much of a threat. With a vast array with crystals available to you, there shouldn’t be a problem taking this kraken down. That said…
The Fight Kiln is unlocked after beating the Fight Caves, completing the quest The Elder Kiln, and sacrificing a fire cape. After the initial sacrifice, you can enter the Kiln indefinitely without a fee. You can teleport there with the Tokkul-Zo, which is automatically awarded after The Elder Kiln.
Har-aken has two parts that’s attackable: the main head with 150,000 LP and a multitude of tentacles that can attack with either Ranged or Magic (7800 LP). The main head can’t attack at all, but the tentacles can hit upwards to 100. Defeating the main head is all that’s required to finish the fight.
You can use any style to attack Har-aken, but Melee and Magic are preferred above all else due to their superior DPS.
Har-aken starts off submerged in the lava, spawning tentacles to attack you. He remains inside for around 30 seconds, unattackable in the meantime. Afterwards, he emerges for around a minute to be attacked freely. He will NEVER spawn at the north side, so keep an eye out for the west, east, and south sides.
The common strategy to kill him before he dives down again is to activate a damage-boosting crystal and use Berserk/Sunshine and just building up to your powerful thresholds. When he dives down again, just kill more tentacles near the south side while you wait for Har-aken to resurface, then go back on him.