I wouldn't call that a fanfic. I'd call that an in-game-battle description, and a... rather badly executed one at that.
First of all - form. Excessive usage of exclamation points, random capitalization, a few typos, (If you want to add an é character, do Alt+0233) and somewhat wonky grammar. This needs work. A LOT of it.
Second - content. Details, man, Details. GOD is in the DETAILS. It's not just shouting commands, faceless characters and vague settings. A good story has to draw you INTO it. To make you SEE the events. FEEL them, in a way. This thing doesn't have any real feeling to it, I regret to say. It's fine in the games, but the games are not fiction.
Put some effort into describing the surroundings, the characters, the creatures involved. The attacks. The /emotions/ involved. Put some sense of dynamics, of emotion, of intensity to it.
"Three Team Magma Grunts and three Team Aqua Grunts started battling in the small, cramped cave." sounds like the beginning of a joke rather than a battle segment. This needs some exposition - where IS the cave, what's its significance (or if you want it to be all mysterious, just the surroundings, to be explained later. Also, if it's so small, how the frell would six different PokéMon and three humans fit in? Also, one-hit KO's are okay ingame, but non-realistic. I tried to work with it here, but, a battle that ends too fast is somewhat boring, to me.
Here's how I'd do it, for instance.
They stood there, three against three, facing their opposite numbers defiantly in a cave that seemed almost too small - cramped, almost - to contain what was to come.
They were only grunts. No one in a position of power told them why they were exactly there - but of course, Team Aqua and Team Magma were sworn enemies, and so, whenever they met, a battle was to come. This would be no exception - Aqua and Magma were about to proverbially cross swords once again. They were all confident, determined to defeat their foes and bring glory to their team. Further their own purpose.
The PokéMon were called out, appearing in six flashes of red. Two Mightyenas snarled at each other, the dark canines baring their teeth threateningly, flattening their bodies and slowly circling each other as if sizing each other up - an intimidating sight to all who beheld it.
A Blaziken, proud and tall, snapped into a fighting stance, blazing wrists flaring up and intense blue-and-golden eyes narrowing as it took in the sight of its opponent - a blue, hulking flippered beast, vaguely amphibian, orange whiskerlike gills portruding from the sides of its face - already standing ready to attack. The Swampert roared at the Blaziken, preparing to use its type advantages against the fire bird - already seeing its victory in its mind before the battle has even begun.
Then, an orange, furry behemoth stepped up into the arena - hooves thundering slowly, clouds of smoke billowing from mountainous humps growing out of its back - a Camerupt. The beast turned its eyes towards a darkened pool of water in the cave, where the sleek, almost mechanical-looking form of a Sharpedo materialized, turning its luminous red eyes towards the Eruption PokéMon and snapping its huge jaws menacingly.
There was a great stillness - the silence before the storm. It was as if the air itself was volatile, preparing for the trainers to let loose the first commands - the first gunshot that would ignite it. PokéMon and Humans, facing off, eyes focused on their enemy.
But it was Magma that blinked first.
"Mightyena! Take Down! Get that Blaziken!" commanded the Aqua Grunt.
"Swampert, Mud-Shot the Blaziken! Together we can take it down before it knows what hit it!"
The large water/ground type reared up, taking a deep breath - and with a roar, spit out a glob of mud at the Blaziken at high velocity - like a bullet leaving a gun. The avian fighter screeched, attempting to dodge the impending mass - but where it dodged, there was the Mightyena - the wolf-like creature leaping-tackling it and knocking the avian down to the ground fiercely - in perfect timing to take the Mud Shot to its back. The sheer power of the hit sent the Mightyena flying into a nearby wall, but it held on. The Blaziken, however, was badly hurt - two powerful attacks at the same time (one of which being super-effective) has left it struggling to get up, Its flames intensifying for a while as its Blaze ability kicked in. But determination alone cannot fuel the body, and soon enough it became clear that the hits that it has taken were too severe.
The Blaziken screeched one final time and relented to unconcsiousness. The Aquas have claimed first blood.
The Blaziken's trainer returned the bird to its PokéBall, clearly dismayed - perhaps a little shocked, though he was wary of showing that to his opponents - or his teammates. He had to walk out of this battle with dignity - if only for Team Magma's sake.
"Get the bastards for me." He almost seemed to spit the words out at his teammates, and walked away from the scene, PokéBall in hand, tattered cloak blowing behind him.
The Aquas were grinning snidely - the battle has barely begun, and already, they were winning!
"Too much for you, eh, Landlubbers?", one of them dared to taunt with a grin.
"Save your breath, water-boy. You're going down faster than the Titanic!" A Magma grunt retorted "Burn before the flames of Team Magma! Camerupt, Flamethrower their Mightyena!"
"And you, Mightyena! Put those teeth into good use and Crunch that Swampert's HEAD off!" the other Magma yelled out, pointing at the Swampert. If his finger was a gun, the Swampert would have had a gaping hole in its forehead.
The Camerupt charged forward, fire in its eyes, reared up and slammed its hooves on the ground, unleashing a stream of fire from its mouth in the direction of the Mightyena, illuminating the cave brightly - a haze of heat within the air. The Mightyena seemed to scream more than yelp as the flames consumed it, attempting to roll out of the center of the flames, but where it went, the flames followed with a slight movement of the Camerupt's head.
"Swampert, DO something!" cried an Aqua grunt frantically as the Mightyena's trainer seemed to freeze, staring at his wolf in horror, unknowing what exactly to do. The Swampert turned towards the Mightyena and prepared to douse the flames with a Water Gun attack - when suddenly its eyes widened in pain and it shrieked as the Magma Mightyena's teeth sank into the back of its neck with vicious force, tearing through its skin into the flesh.
The Swampert lost control of its attack, trying to shake the canine away with frantic movements of its head and body. The Mightyena latched on, tightening its grip on the Swampert. Thick blood began rivuleting from the wound, slowly, onto the Mightyena's muzzle as the Swampert's cries of anguish melted into the Aqua Mightyena's - as the flamethrower finally relented, leaving the badly burned canine on the ground, fur charred, still and unmoving. The gigantic Mud Fish did the only thing it could... Walked backwards and slammed powerfully several times into a wall, attempting to shake the canine off. The Canine yelped through clenched teeth as the rocky surface of the cave wall slammed painfully into its back, battering its form - But the creature was loyal to its trainer and fought on valiantly, until finally, the Swampert blacked out from the pain, collapsing to the ground with a mighty thud. The Mightyena rolled off, battered, exhausted, but with a happy canine grin upon its beaten face. It has /won/.
As the two canines and the mud fish were returned to their PokéBalls, their trainers leaving the arena, it was clear to the two remaining combatants that this was it - the big one. The Sharpedo, swimming in rapid circles in its pond, locked its eyes on the Camerupt, which in turn turned to face the Shark PokéMon in preparation for attack.
Two were down on each side. Now, two remained to determine which would be superior.
To sum things up - 'This is my first fanfic' is no excuse for anything. We were all new to writing once - and we all had our stumbles - but as long as you are willing to learn and improve, you might become good at this in the future.
StellarWind Elsydeon, signing off.
