The Fire Giant is located in the Mountaintops of the Giants, he is massive so you will see him in the large open area.
He has 2 phases, he is very slow but hits hard. He will use his shield, throw rocks and smash you with his fists. You can ride your horse Torrent and attack his legs and ride away if you are struggling to slowly chip him down. You can also summon your ashes to help in this mandatory boss fight, the Fire Giant will aim at them allowing you to get some extra damage in.
Phase 2 will begin and he will attack you with many fireballs. Because he is a giant he is very slow, I suggest not getting too greedy as in Phase 2 he rolls a lot which can damage you greatly. Use Torrent to your advantage here!