On the behalf of all fire mages in your setting, allow me to say two simple words.
So, you don’t want any metallic dust in your face ever for any reason ever, no thank you, I’m fine. Normally we’re talking eye damage, possible blindness, lung damage (could be fatal), not to mention possible abrasion of the skin, later illnesses developed by exposure to various metallic dusts, and of course metal poisoning. All of which would suck.
And none of which matter at all in this case.
Magnesium burns very hot. As in 2,500 K, 2,200 C, or 4,000 F kind of hot, and those are rather lowball estimates. If a cloud of magnesium dust is ignited, everything in the cloud is dead, everything close to the cloud is dead, and if something in the cloud is completely immune to heat, it doesn’t matter because all the oxygen in the area, and probably in said heat immune creature’s lungs, is going to be gone. No oxygen is normally kind of a downer because things tend to die very quickly when deprived of said element. But wait, it gets worse. Within the cloud, death will likely be nearly instant because any living thing will probably inhale superheated air and ignited magnesium dust burning at 2,500 K. Not exactly healthy for your insides.
If you are outside the instant death range but still relatively close, you’re still dead because of severe burns causing dehydration and infection, and that’s if you don’t die from shock first. Outside of that range you are still burnt and blind, possibly permanently, from the intense heat and light. Finally, you will also ignite anything that is even remotely flammable, and depending on where the fight is taking place, that could be very very bad as well.
Also, in modern terms this would be considered a very serious war crime.
Your metal mages would need to remember, however, that the fire mage could ignite the magnesium before it reaches them, which could very well flash fry your metal mage and all of his friends.
Now, I’ll probably regret informing you of this next part, but here goes anyway. Your metal mages could actually do much, much, much worse.
Enter aluminum and iron oxide. Mix it together just so and give it a little ignition source like, oh I dunno, a fire mage, and you get thermite. Now, many sources say thermite also burns around 2,500 K, but it has one tiny, little, easily overlooked property that might be kind of important in this scenario.
It brings its own oxygen.
Not sure what this means, WELL allow me to be the person who haunts your nightmares forever because it means you can’t extinguish thermite once it starts burning.
- Water? Nope, we use it for welding underwater.
- Vacuum? Nope, it has its own fuel and oxidizer and will burn until one of them is gone.
- Cold? You would have to instantly chill the entire 2,500 K fireball so low that it stops oxidizing, so good luck.
Congratulations, you’ve just created one man terror weapons that can be scaled up to, basically arbitrary size that are nearly impossible to survive. For added horror, if you do this inside any enclosed space, you get a low tech thermobaric bomb that kills from heat, the pressure wave, and the following lack of air that can last for minutes depending on the size of the bomb.
Happy hunting you demented war criminal.