How to Defeat Fire Giant
Do this. No spirits. No mount.
Stand under him. Use Meteorite, not Meteorite of Astel.
Since you’re lower level, perhaps it would take 3 minutes instead of 2.
And ignore Rennala’s Full Moon and Terra Magica. I was a noob and didn’t yet understand that Gravity Sorceries didn’t benefit from those debuffs. Also, Fire Giant has 0 physical and magical damage negation, so it wouldn’t have mattered anyway.
Note: You can also try using the weapon art of wing of astel. Since he is such a big target, he gets hit by a lot of the explosions. To increase survivability, you can put on the fire absorption talisman and eat fireproof dried liver.