Multiattack.The giant makes two attacks, using Flame Sword or Hammer Throw in any combination.
Flame Sword.Melee Attack Roll: +11, reach 10 ft. Hit: 21 (4d6 + 7) Slashing damage plus 10 (3d6) Fire damage.
Hammer Throw.Ranged Attack Roll: +11, range 60/240 ft. Hit: 23 (3d10 + 7) Bludgeoning damage plus 4 (1d8) Fire damage, and the target is pushed up to 15 feet straight away from the giant and has Disadvantage on the next attack roll it makes before the end of its next turn.
Features
Titan's Momentum — Passive: Once per scene, the fire giant can be spotlighted twice during a GM turn. Spend Fear as usual to spotlight them.
Hammer Throw — Action: Mark a Stress to hurl a hammer at a target within Far range. On a success, deal 3d12+2 physical damage. The target is knocked back to Close range and has disadvantage on their next action roll.
Fire-Touched — Passive: The fire giant takes no damage from fire, whether physical or magic. Additionally, its attacks deal an extra 1d6 magic damage, added to the attack’s total.
Designer's note
As large and powerful adversaries, fire giants are frightening on the battlefield. Beyond that, though, they are perceptive and wiser than the average person would expect.
Why Bruiser?
Fire giants are large and powerful creatures in 5e. They have a lot of health and can deal a lot of damage. That pointed me toward either a Bruiser or a Solo. With its limited abilities, a Bruiser felt more correct, since Solos are often a bit more complex.
Wow, this guy hits hard
In 5e, the fire giant is powerful and hard-hitting; it’s at the upper end of the Tier 2 conversion range. That, plus being a Bruiser, means it should hit hard. It’s probably better thought of as Tier 2.5 than a straight Tier 2, showing up closer to level 3 or 4 than level 2.
Titan’s Momentum
In 5e, the fire giant has Multiattack (two attacks); Titan’s Momentum is my compromise instead of giving it Relentless (2), which seemed too powerful here. This lets it really pack a punch once per scene without overwhelming lower-level adventurers too much.