Flame Phantoms are bosses that can appear during certain side content. They cannot be harmed directly until you fill their stun bar. When this occurs, they'll dematerialise, and you'll need to smash the enchanted pillars in the immediate area to take chunks out of a Phantom's health.
Phantoms take on an amorphous form for most of the fight. Their only weak point is the obvious rune core at their centre. You can hit this with both melee and ranged attacks, although the latter is the safer option. This is because Phantoms are capable of unleashing powerful unblockable attacks if you're too close.
Once you've dealt enough damage to the core, Phantoms will dematerialise, as mentioned. At that point, it's just a case of finding their pillars and dealing as much damage as you can before the Phantom returns. Pillars cannot be damaged when the Phantom is on the battlefield.
Fortunately, Phantoms don't have that many attacks of their own. Play close attention to when they're using an unblockable, and simply guard or parry the rest of its techniques. Stay mobile if possible, all while hitting that core when you can.
How many Flame Phantoms have you smashed into submission? Be sure to check out our fullGod of War Ragnarok guide for information on how to beat every enemy, and much more.