So it's in a sword and sorcery universe. The POV character is a band of mercenaries defending a pass. And attacking that pass is a mass of demons, zombies, and etc, with a sorcerer behind them, trying to break through. It's a bad situation. And then a sun is born. They're rescued by a massive phoenix-like creature (which is also a human and is also their ally). The effect is the equivalent of an artillery barrage in close quarters, fireballs raining from the sky like rain while leaving them untouched. The sorcerer tries to fight/ run, but his magic is incinerated, and he just plain dies. Now in this universe, magic isn't something friendly or close by. Most people live like iron age people, and magic is often dangerous, slow, ritualistic, and wielded by less scrupulous individuals. It takes time to cast, needs verbal, somatic, and material components, and generally wrecks the sanity of those who witness it, sending most men into fleeing terror. Those who see it feel that magic has an unnatural aura. Generally speaking it's not very user friendly or PR friendly. Think 'Conan the Barbarian' instead of 'D&D'. Generally speaking I have an outline, with descriptions of 'felt like he was in a furnace' and 'a sun rose'. Descriptions of various monsters burning to ash, the air catching fire, etc. Men throwing themselves into fear and worship. But I'm more talking about internal narration. What would be the best internal narration. Should I have him narrating about all the different ways he's seen sorcery before, but nothing like this? Should I have him reminisence of the various times he's seen fire burn things, but not monster? Should I have him be focused on his soldiers around him? Or should I just give him a religious experience?
Look into ideas about the Sublime from the 19th century. There are 2 different strains of thought about it, the philosophical and the literary. You probably want the literary mostly, but if you look into Burke's philosophical ideas on wikipedia (and a few others as well) you'll find some great stuff.
Difficult to answer. What's most important to the character? Which line of thought is most likely to telegraph the correct emotion to the reader? Fear and awe and relatively simple things. And they're to be expected when witnessing death, fire, magical exposions, etc. The reader (in theory) should already be on a similar wavelength, so getting the "awe" message across should be straightforward. I'd be more worried about overdoing it. Given the extreme scenario, you shouldn't need more than a few lines of interior monologue to get the point across.
