In the Dark Souls 3 kingdom of Lothric, the Ashen One must confront the twin princes Lothric and Lorian to reach the final area of the game, the Kiln of the First Flame. Though, the king of this expansive land is not quite out of the picture despite no longer having anything to do with the throne.
Oceiros, the Consumed King can be found in the aptly named Consumed King's Garden located not far from the bonfire in the large room at the High Wall of Lothric where the player fought Dancer of the Boreal Valley. Driven mad and having transformed into a dragon-like being, this monarch obsessed with the power of dragons and sorceries is a strange boss, though one that can be fought and defeated in similar manners that one would slay a dragon.
Melee Strategy #1 - Use A Lightning Weapon
One of Oceiros' biggest weaknesses is lightning damage, a result of becoming more dragon than human. As such, any lightning weapon will do sufficient damage on this boss if upgraded a few times. Players intent on slaying the mad king with a melee weapon should try to stick to his flanks and tail, as these are the areas in both phase 1 and phase 2 where Oceiros will have a hard time hitting the Ashen One with most of his attacks.
A great option, if the player has for some reason already cleared Archdragon Peak, is the Dragonslayer Greataxe for its immensely deadly amount of physical and lighting attack that can even interrupt some of the pseudo-dragon's attacks due to the huge poise damage that it can inflict. It is worth noting that if one hurts the boss enough in a short period of time, Oceiros will get staggered and will become vulnerable to a critical attack.
Melee Strategy #2 - Use A Frost Weapon
Frost weapons are pretty deadly in a variety of situations and this boss fight is one of them, for Oceiros' only other main elemental weakness is this status ailment. Like with other melee strategies used again this draconic enemy, stay close to the sides of the consumed king's legs in his first, bipedal phase and essentially underneath his tail and hind legs during his second, quadrupedal phase.
The faster ones, like the Irithyll Straight Sword or the Pontiff Knight Curved Sword, would be more consistent at building up Oceiros' frostbite bar, however, Vordt's Great Hammer has excellent interrupting power in addition to the highest natural single-hit build-up of frost in the game.
Miracle Strategy - Use Lightning Spells
For much the same reason lightning melee weapons are effective against the boss, miracles that possess such a damage type are another obvious choice with which to fight Oceiros. However, do not try to cast these from a distance, as the consumed king will likely be able to close the gap or cast his own, faster spells before the player can incant any miracles.
Instead, try to stay in somewhat of the same positions where a melee build would be, though not quite as close so that Oceiros' tail swipe in his phase 1, and his spin attack in phase 2, won't hit the player. Great miracles to use here would be Lightning Spear (or one of its more powerful variants), Lightning Stake (especially effective if cast from underneath Oceiros during phase 2), and Lightning Arrow (for fast and reliable damage).
Sorcery Strategy - Use Snap Freeze & Dark Spells
It would be a bad idea to fight Oceiros with magic-based sorceries, as he is highly resistant to them, therefore sorcerer builds will have to rely on spells that deal different kinds of damage. Snap Freeze is a great option, as it can target the fake dragon's frost weakness.
Dark sorceries are also a nice choice, for they can deal quite a lot of damage, particularly the highest form of Deep Soul, known as Great Soul Dregs, as it has a huge hitbox that makes it hard to miss even erratic targets like Oceiros' second phase.
Pyromancy Strategy - Use Projectile Spells
It may be tempting to unleash huge columns of flame or other fancy pyromancies on Oceiros, the Consumed King, however, he won't be likely to stay still while one casts these big spells, especially in his wild second phase. Casting variants of the Fire Orb spell, such as Great Chaos Fire Orb and Black Fire Orb, at the boss while near his legs (though not too close) will deliver reliable damage at moderate safety.
The imitation paledrake is not weak to fire damage or dark damage, however, he does not resist them either. For this reason, it is best to take a more cautious approach, as one will not be able to burn up the boss's HP quite as quickly as with other offensive tactics.