The God-Devouring Serpent is one of a few Elden Ring bosses built around what could be uncharitably called a gimmick. It requires — or at least forcefully suggests — the use of a special weapon to complete. This is a tradition that stretches at least as far back as the Storm King boss in Demon's Souls. It's also a surprise two-phase fight that asks a lot of focus of players. Here's how to beat the boss.
God-Devouring Serpent Phase One: Using the Serpent-Hunter
When you first enter the arena in which you'll fight the God-Devouring Serpent, you'll be able to pick up the Serpent-Hunter. This Great Spear is a decent weapon generally, but it has a special property that reveals itself when fighting the God-Devouring Serpent: Its attacks produce a long blade of light that extend its range and deal extra damage against the boss. Its Ash of War is also useful, charging up a strong attack and optional follow up that will stagger the Serpent.
You'll want to use this weapon throughout the fight, even after the boss enters its second phase. That's true even if you're primarily a magic user. Magic simply isn't very effective against the Serpent.
When you equip the Serpent-Hunter, make sure you don't exceed your equipment load of choice, and adjust your armor accordingly to keep your rolls at the speed you prefer.
The first phase of the boss fight isn't terribly difficult. The Serpent only has a few attacks, and most (not all) are easily dodged. Its snake bite, in which the Serpent lunges at you from one side and often follows up with a swipe from the opposite direction, can be dodged by rolling into the lunges. Its grab attack is telegraphed by a long windup of its head, and you can dodge straight into it. Both allow for big punishes, potentially even full, three-hit light attack combos.
The Serpent has three additional ranged attacks. When it goes to spit poison, you can dodge by sprinting to one side or the other and rolling in that same direction when the poison is about to reach you. When it slams its head into the ground and launches magma at you, you can roll forward through it. When it starts to thrash violently and shoot magma randomly, just back up until the attack ends.
The only truly troublesome attack the Serpent has is when it plunges its head into the ground and erupts back out, causing a magma eruption under your feet. This attack can theoretically be dodged by sprinting perpendicular to the Serpent and jumping or rolling just before the eruption, but you're often better off just trying to block.
God-Devouring Serpent Phase Two: Rykard, Lord of Blasphemy
When you deplete the Serpent's health bar, it will reveal itself to be the Demi-God Rykard, Lord of Blasphemy. You'll still want to use the Serpent-Hunter weapon for this fight, but it will be a more protracted fight than the first phase.
Most of Rykard's attacks are fairly straightforward to dodge. His sword combo swings from left to right, then right to left, then stabs forward. The second swing has a longer delay than you might expect, hitting the ground and stutter-stepping before going in for the full swing. Dodge into the sword each time. This procedure is the same for when Rykard mixes his snake head into the combo.
Rykard's stab combo has him hold his sword to one side, then stab the ground near you three times, with an optional fourth attack that rips the sword from the ground and causing an eruption. You can dodge to the side to avoid all these hits.
If you see Rykard flip his head to the left, be ready for his serpent head to lunge at you, and dodge to the left to avoid it.
Rykard can also jump forward or backward, sending magma flying in all directions, and conjure a flaming skull that creates a flaming trail as it flies forward. Sprint perpendicular to these attacks and you should avoid them fairly easily.
Rykard's big, show-stopping attack is called Rykard's Rancor. At some point in the fight, he'll raise his sword above his head and start channeling. You can get some free hits in while this happens, but stay on your toes, as a bunch of flaming skulls will soon begin to rain from the sky and chase you. Impact with them will cause them to explode, and they'll keep coming for quite some time, even as Rykard stops channeling. You'll need to sprint away from these skulls or dodge through them as they explode.
If Rykard isn't dead by the time the skulls stop appearing, he'll start channeling another attack. More opportunities for free hits, but watch out for his sword, as it will come crashing down directly in front of him and create an explosion. You can sprint far to the side or, just as the blade makes contact with the ground, roll to the side.
If you want to make this fight as easy as possible, using a Mimic Tear summon equipped with the Serpent-Hunter will effectively halve the duration of the fight.