Where is the Magma Wyrm
The Magma Wyrm can be found in the depths of Gael Tunnel, an optional dungeon that straddles the boundary between Limgrave and Caelid. It has two entrances, the rear entrance opening in Limgrave and the main entrance which you can find in Caelid. Since the rear entrance doesn’t allow you to fully explore the location (a door which can only be opened from the other side blocks progression), it’s considered a Caelid location.
To reach it, you can use the Spiritsprings north of the Third Church of Marika to ascend the cliffs, then follow the road northeast (be wary of the Smoldering Church, as Anastasia Tarnished Eater lurks nearby) to reach a wall separating Limgrave and Caelid. At this wall turn south and jump on some rocks and ride south, drop down a ledge to find a graveyard, drop down another two ledges and turn east to find another entrance to the Gael Tunnel.
Once inside you’ll have to make your way down a pit, dropping down ledges until you reach the bottom, where you’ll find the Gael Tunnel Site of Grace. From here you’ll need to make your way through the caverns beyond - including one filled with numerous Miners and Radahn Soldiers - climb down a ladder guarded by more Miners and Radahn Soldiers, then progress through some tunnels until you find some large wooden doors, beyond which dwells the Magma Wyrm.
This is a rather stunted version of the Magma Wyrm, lacking a definitive second phase that some other Magma Wyrms will possess. That being said, that doesn’t mean this boss is a pushover - it’s still a Caelid-level boss (albeit, one of the easier ones), and while it won’t resort to standing upright, the limited space you have to fight can make its first phase attacks dangerous enough. It deals fire damage with many of its attacks, so having some form of fire resistance will help.
The Magma Wyrm will rear back with its sword arm and perform a diagonal slash. Very easy to dodge, but it may be performed multiple times in succession, especially if the first slash hits.
Smash and Hook
The Magma Wyrm will smash the ground with the flat of its blade, and after a brief delay, the dragon will drag the sword back towards itself along the ground. Both the initial flat-blade impact and the hooking drag afterwards can deal damage.
The Magma Wyrm will stand upright, grab its sword with both hands, then slam the blade into the ground. Deals massive damage and has moderate AoE.
The Magma Wyrm will belch a glob of magma. It will signal this attack by tossing back its head and regurgitating magma, which can be seen in its mouth before spitting it. The magma will stick to the ground for a while, and will burn you if you step in it.
The Magma Wyrm will rear its head back and regurgitate magma, after which it’ll stomp forward, spewing magma in front of it as it goes. Has very minor tracking, but leaves a large slick of magma on the ground, which will damage you if you touch it. You can avoid this attack by simply strafing around the dragon.
Magma Over-the-Shoulder Belch
The Magma Wyrm will look over its shoulder and spew a torrent of magma. It will perform this attack when the player is to the side of the Magma Wyrm. The magma will stick to the ground for a while, and will burn you if you step in it.
The Magma Wyrm will rear back and perform a massive sweep with its sword. Performs this attack when the player is behind or to the side of the Magma Wyrm. Deals massive damage and strikes a wide area.
Armed with a massive sword, magma breath and a bad attitude, the Magma Wyrm is no trivial foe, especially when you’re forced to fight it in its lair, which is on the small side, considering the range of some of its attacks.
The Magma Wyrm’s offense can largely be split into two categories: sword strikes and breath attacks. All of its attacks deal moderate to heavy damage, and while some of its sword attacks (especially its Sword Sweep attack) have great range, its magma breath is the real concern, as it leaves persistent pools of magma which will deal damage if you touch them. Given the size of the arena and the sheer amount of magma the Magma Wyrm can spew, this can quickly become a real threat, especially if the Magma Wyrm decides to perform its Magma Charge attack in succession. Your summons are also generally too stupid to avoid the magma, so be wary when using them - they may not perform as well here as in other fights.
Fortunately the Magma Wyrm is fairly lethargic and telegraphs its attacks. Once you learn its tells you can easily dodge most of its offense and retaliate. This fight is easier if you have fire resistance and can perform ranged attacks, even if the Magma Wyrm is resistant to spells - it simply isn’t offensively dynamic enough to effectively counter magic.
The Magma Wyrm is, fittingly enough, highly resistant to fire damage. It also has moderate resistances to physical (slash), magic, lightning and holy damage, being only particularly receptive to physical (strike) and physical (pierce) damage.