At RL1 (almost) every little twitch from Fire Giant can one shot you and he has a truly staggering health pool of 43,263 HP (for comparison Morgott has 10,399 HP). Phase 2 is particularly dangerous since the fire attacks are so far reaching.
But he has giant -- pun intended -- attack opportunities in phase 1 and you can set up a stagger for the opener of phase 2 and then kill him right then and there or he has just a sliver of HP left.
It also heavily depends on the weapon choice for this fight: Star Fists -- my favorite -- absolutely wrecks him. Alabaster Lord's Sword is supposed to be even better.