Your prose needs some work. First and foremost you're containing too many disconnected actions within a single paragraph.
Heru-Ur strode up behind the Spireguard line, and those who turned at the sound of his approached let out a cheer, taking heart from his presence. A tense few seconds passed as they waited for the Wolves to close. For a moment, it was almost peaceful, reflected Heru-Ur, admiring the way smoke drifted across the street. The serenity, however, was spoiled by clamour of the Wolves’ advance. Suddenly, without warning, the Wolves burst through the smokescreen and hell, as the old saying went, broke loose.
There are three separate sets of actions, walking up and down the line, a brief moment of serenity, and the "hell breaking loose." They would function better as either two or three disparate paragraphs. It would also be a good moment for added characterization of your protagonist or the foe in a way that was active rather than narrative.
Even when your thoughts are connected you tend to jumble them together. For example.
The Custodian marched right up to Heru-Ur and levelled his spear. Heru-Ur brought his melta to bare, then cursed himself for his lapse in judgment – the weapon was useless for anything other than bludgeoning now. Grunting in frustration, he took a step towards the Custodian. The Custodian sidestepped and leapt forwards, faster than Heru-Ur had anticipated. The spear was a blur of silver and gold, striking Heru-Ur thrice before he could react. Every counter he made was too slow, every riposte met thin air. Dents and cracks began to form in his hull, the Custodian exploiting every weakness and stress inflicted in Heru-Ur’s duel with the dreadnought.
This is a very abrupt and jumbled paragraph. There is a lot of action but not a lot of chance for the reader to catch up with it. Honestly the vast majority of your paragraphs suffer from this. You're trying too hard to compress a lot of action into a very small space.
That being said this is not a bad story. It's actually quite a good one, but your writing needs a bit of polish. I would suggest reading your story out loud to yourself and seeing where you pause for breath. The natural cadence of a paragraph should comfortably fall so that a break between paragraphs and where you would naturally pause are roughly the same.