In a duel, one of the fighter is a fire mage who is capable of producing heat surges in a wave-like pattern. There are flames coming along with it, but it's purely for aethestics and has no physical impact whatsoever (as far as I know it's really hard to ignite air itself anyway). That way, you can imagine your favorite comic/movie/anime/book arsonist, so it's cooler. Well, sort of.
For the sake and accuracy of this question, this wave raises the air temperature almost instantaneously by 1 000 °C (~1 800 °F), then goes back down to the point it was as the wave fades away. Note that this heat peak only affects gas's temperature, not solid materials like an human body or their clothes. It can however "burn" the air inside one's lung if aimed properly at the mouth or nose. Solid components keep the heat they received through this surge, meaning they continue to burn if they have started to.
Of course, the longer you are under such heat, the more roasted you will smell if you haven't started to burn to ashes already, due to how heat transfer works. But if an human wearing cotton clothes is subject to a single fireball for 1 second of exposure, how bad would really be the damage? Would their clothes/skin start to burn? If not, how long/how many fireballs would you need to make this happen?
As a bonus question, and in order to make it more realistically dangerous, what would be the outcome if the heat spike also affects solid materials on that split second?