As an aspiring professional writer there are a few litttle things that i've noticed here which i think you might want to adress.
It came at him again, its axe throwing his blade to the ground. He leapt out of the way of a decapitating blow, rolled from a swing that would have bisected him from brain to balls. He lost sight of his blade in the swirling one on one. He grabbed for his bolt pistol, as he done so the ork smashed him from his feet. He hit the ground hard, mud squelched under his black armoured bulk. He lifted his bolt pistol.
Watch your repetition, you've used a lot of him, he, and his throughout the story which adds an air of mystery but will get very repetative and start to read a little like a shopping list. Above is the worse case in your story. In 4 and a half lines you have used 6 hes, 6 his and 3 hims.
The daemonette recoiled as he looked it in the eye, fiery light emitted from his eyes.
This was a minor repetition, i would consider changing the last word for something else.
maybe something like "fiery light flooded from the well of loathing within his soul."
You also don't mention a change of scenary after you have introduced the sparring ring, consequently it isn't obvious wether the warrior is still in the training ring or not. I assumed not but the lack of backing automatically makes the reader envision each battle as being in the same place.
Aside from that i really like the whole theme of the story, the proving of worth to the emperor and such. Your pace during the combat sequences is also very good and drags the reader in. Keep it up.