Blue glared across the blank slate that was the battlefield. Blue had prepared it's forces, once again, for yet another battle.
I’d eliminate either “once again” or “yet another”. They both tell basically the same info. Take a look.
“Blue had prepared it's forces for yet another battle.”
“Blue had prepared it's forces, once again, for battle.”
Either tells the reader the same thing: that this force has been on combat before (seemingly many times before).
Try starting either the first or second sentence with a different noun. It’s a good idea to slightly alter your sentence structure.
The troops stood ready and in formation. An entire repertoire had been arranged for this battle. X's and Y's, ='s and =/='s, -'s and +'s. From every corner of Blue's mind had they marched to this battlefield to fight once again with Blue's archenemy, Red.
Bit of confusion here. Is blue a commander, the army, a specific character in this army, or a combination of the above? It’s fine if all of these are true, just make sure you clearly specify which of these you mean.
Last time, they had suffered a terrible defeat at the hand of Red. This time, however, they would win.
Ah, some history. Let us in on a little of that terrible defeat. Just a few sentences should do. Maybe a description of the grisly state of the battlefield that day would be sufficient.
The second sentence needs to be clearer. Is this a statement of things to come, or the opinion of the narrator?
They had occupied the most important bottlenecks and the highest points on the nearly uniformly white battlefield. Difficult choices had been made and questions had been answered. It was now time for the showdown.
Hmmm, not sure that I like the word “uniformly” in there. It seems redundant. That is to say that the phrase “nearly white” gives practically the same idea as “nearly uniformly white”.
I’m also left with a few questions: if the field is almost entirely white, what did the rest of it look like? What were the difficult choices that had been made?
Red appeared at the horizon. Lines, circles and squares had gathered around him. The very sight of them was one that would make men flee, but his forces knew better. They stood a chance.
The “him” here is unclear. As stated it looks like you’re identifying Red as the commander, stating that those shapes have gathered around them, that the sight of those shapes would be enough to make someone who saw them run away, that Red’s forces know better, and that Red’s forces think they stand a chance.
I’m guessing that you meant “his forces” to indicate blue, but the phrasing doesn’t make this absolutely certain.
For a few moments, the battlefield was silent. Nervously, the troops stared at eachother, wondering who would kill who and who would survive.
Just a couple of minor things. “Each other” is two separate words. The second “who” needs to be “whom”
Gotta love the word “whom”. It’s one of the few artifacts we have from early English back when the language was mostly Germanic. It’s kinda like the Coelacanth of this language.
The silence was broken by a thundering blast as Red stomped on the battlefield. Red's troops charged and swarmed and swirled around Blue's carefully placed positions. The soldiers of Red spilled inkt as if it were blood when they clashed with Blue's soldiers.
You can get rid of the last line after “blood”. We can assume that the Reds are slashing up Blue soldiers. No other targets have been given.
The swarms of Red were unendless.
UN endless would mean that they had an end.
Some of Blue's formations cracked under the constant pressure of Red's thundering waves, other seemed almost immune. The battlefield was soon coloured with blood both red and blue. Almost everywhere, the colours had mixed in to a purplish shade, but on a few rare places, the colour of Blue has still survived and was yet untouched by Red.
Try to avoid general language like “almost” and “seemed”. When you use “seemed” it could be taken to mean that something appeared, but was not, the case. So in this instance it suggests that some of Blue’s formations appeared, but were not, immune to the attack. General terms also make descriptions feel weak. It’s hard to really picture a scene when it sounds like the narrator doesn’t have a clear view of what is going on.
The battle lasted no longer then a few minutes, and it ended suddenly. The battlefield, once a perfect field of peacefulness, was now a ravaged place. A twisting and turning pattern of red and blue buried the once white earth, symbols and letters, the corpses of heroic soldiers, were spread all around.
The teacher carefully examined the remnants of the battlefield, searching for mistakes he had passed over during the battle.
With a sigh, he once again wielded his red weapon, and he spilled yet more blood as he wrote on the very bottom of the sheet of paper;
"Come see me after class!"
I think this would be stronger as “…the corpses of heroic symbols and letters…”. We already know that those things are the soldiers here, and the sentence flows a bit easier without the extra phrase.