The final boss of the Mountaintops of the Giants, the Fire Giant is a challenge in large part because its enormosity makes it hard to see what it’s doing. Otherwise, it’s actually a fairly straightforward fight, albeit long — read on to see how to take this big fella down and gain access to the Forge of the Giants.
General Tips
The boss’s left leg (the one without a metal bracelet) is its most vulnerable spot in Phase 1 — in Phase 2, the boss’s wrists become its weak spot (and the eye in its stomach, but that’s pretty hard to get to).
Being behind the giant is the safest place during Phase 1. During Phase 2, this is still true, although the AOE fire attacks make it less safe than during phase 1.
Naturally, the Fire Giant is very resistant to Fire, and he also has moderate Holy damage resistance. Surprisingly, he’s fairly resistant to Frostbite, although it does still work on him.
Bleed is an effective status effect to inflict on this boss, since he has such a large HP pool. If you use this technique, consider pairing it with the Mimic Tear Ashes.
Strategy
You’ll want to stay mounted for this whole fight. It’s also best to leave the camera unlocked so that you can pay attention to what the Fire Giant is doing. If you find you aren’t doing enough damage to this boss, consider respeccing into a Bleed (arcane) build — this boss’s large health pool is negated by the %HP damage bleed does. It’s also a very good idea to craft your strongest Raisins for Torrent and put them in your toolkit, as many of the Fire Giants attacks will injure poor Torrent, and keeping your mount healthy is essential throughout this fight.
For summons, Latenna is a great choice (isn’t she always), as you can summon her very far away from the boss, and she’ll distract him and mess up his AI without ever really being in danger. If you don’t want to use Latenna, Black Knife Tiche is also viable thanks to her %HP damage. The Mimic Tear is also a good choice, especially with builds that aim to proc a status effect.
Mage builds should memorize the Adula’s Moonblade sorcery — it can be used to hit multiple body parts of the boss at once, and will therefore do very good damage in both phases of the fight.
Phase 1
Ride up from the entrance a bit (assuming you came in from the northern one) so that you’re on a part of the slope that has a clear view, and then summon Latenna. Next, ride towards the fire giant with the goal of reaching his left heel — the one on the leg that has a bone splint on it. While approaching the giant, he’ll often do the Snow Scoop move. It’s easy to see coming, and it is possible to avoid: as he brings up the disc to throw snow at you, press the crouch button to dismount Torrent. (Note that if you’re too close, this technique won’t work, since the disc will still hit you).
Avoiding the Snow Scoop
Stand at the left heel and smack it until the rope comes off — this will stagger the giant momentarily, and you can take this chance to get some more free damage in (still on the same heel). For the rest of Phase 1, you’ll want to spend as much time as possible on this heel, attacking. You can use this strategy as a ranged character too, and you’ll be able to stand further back from his heel, which makes avoiding his attacks even easier.
Keep an eye on your stamina, since you’ll want enough to dash away if necessary. Sometimes, the giant will turn towards you — when this happens, immediately tap the sprint key and start riding away (in the direction he is turning to face), or you may get smacked/crushed by his Disc. Otherwise, being behind the heel will allow you to avoid all the giant’s attacks — if he does his Roll move, it won’t hit you as long as you’re behind his heel. The Stomp attacks look a lot like the Disc moves in that he’ll often turn towards you, but he raises his foot either way and therefore they are quite easy to avoid.
This pretty much sums up Phase 1
Sometimes he’ll reach into the mouth on his stomach and pull out a Homing Fireball; these follow you, but so slowly that they’re no threat at all unless you aren’t paying attention. Ideally, you should trigger these when you’re safe (after the giant rolls away, for example) by sprinting near them with Torrent — they’ll explode after a brief delay, and coat the ground below them in flames. The giant also has a regular Fireball attack that can be dodged by dashing perpendicular to the giant.
Triggering and dodging Fireballs
Additionally, sometimes the fire giant will call a Firestorm that comes out of the ground — you’ll know this is coming when small fires appear around him. Simply ride away until there’s no fire near you, or if you want to be greedy, you can ride through the fire patches and attempt to attack the boss.
Phase 2
After a classic FromSoft-style cutscene, you’ll fight a new version of the Fire Giant, who doesn’t move as much but has more AOE spells, including a fairly dangerous Fire Spew that acts a lot like dragon breath attacks. He’ll also cast a much wider version of the Firestorm, as well as an Eruption, where he leans back and spews molten rocks out of his stomach. For all of these moves, riding far away is the best counter — the only exception is the Fire Spew, which can be avoided by getting behind the boss as well. Note that he’ll also continue casting both types of Fireball.
In phase 2, his vulnerable spots become his wrists and the eye on his chest (which is rarely targetable — only when he lies on his side).
Still, staying behind him is the safest option if you don’t want to risk taking damage, especially because he still does his Roll attack. He’ll also Swipe toward the ground with his hands as his frontal melee attack (now that his disc is gone) — this can be avoided by being far enough away, or, if you’re very close, by dashing towards the center of the boss.
Really, your best bet is to split the difference between playing it safe and trying to do damage — hang out at his heel and attack it, but when he rolls away, you can try to get some hits on his wrist as you make your way around to his heel again.
This boss fight can be long and frustrating, but don’t give up! Once you learn the rhythm of the fight, it becomes much easier. Don’t forget to take breaks, and if you really get stuck, sleep on it, let those neurons form stronger connections, and come back fresh. (Also consider respeccing to 60 Vigor so you don’t get one-shot.)
Unabashed FromSoftware fanboy still learning to take his time with games (and everything else, really). The time he doesn't spend on games is spent on music, books, or occasionally going outside.
Glad the guide was helpful! Fire Giant is definitely one of those bosses that’s so different from every other fight that it can be tricky to find a strategy that works.
180,000 
Thanks! This guide helped especially the adult moon blade suggestion I have been trying for days to find a useful enough sort ery.
Glad the guide was helpful! Fire Giant is definitely one of those bosses that’s so different from every other fight that it can be tricky to find a strategy that works.