"Pikachu's tail became hard as steel. Pikachu jumped into the air and spun around creating momentum in his tail movement. As Pikachu pulled out one last spin it impacted it's tail with venasaur's head. Venausaur's pupils dialated completely and it fell flat on it's belly, completely unconcious." Now I just came up with that on the spot so there are probably sentence structure errors and details missing but you get the idea.
To add, it's all about getting the reader to actually see and feel what's going on as if they're the characters or right there in the front row. Saying, for example, that Pikachu's building up momentum really doesn't tell the reader much. It's a start, but you've really got to go all the way. For example, not only should you say that Pikachu jumped up, say he did it with a swift movement, maybe to the point where he looked like a yellow blur. Say the tail began to glow white. Say Pikachu crouched down as the muscles of his tail clenched and stiffened. The reader can see and feel all of those kinds of descriptions, and that's really what you want to get at because the battles of a Pokémon fic are usually some of the most exciting or intense moments of the story because that's a lot of what the Pokémon world is all about.
Additionally, I'd suggest avoiding scientific terms such as "creating momentum in its tail movement." You want to aim at simple description because elaborate ones -- ones where you sound like a textbook, at least -- tend to soften the description by making it a bit more humorous than it should be. That and the reader can easily picture something like a swift turn. Something like "creating momentum" is actually somewhat vague on its own (because there's different ways you can do that), and if you
don't have it on its own, then you end up being redundant because you're probably tacking it onto another movement description (so it's like two in a row).
So, the first tip is description. Keep it in simple terms but make absolutely sure your reader can picture or feel what's going on in their heads. Reaction's good too, as Ebon pointed out. You just need to build up to the reaction so the reader can flinch when he reads what happens too.
Second tip is do not have a battle end in one or two moves unless you've got a good reason for doing it. For example, if your character's stupid enough to try to take on Ho-oh, feel free to have the bird roast the trainer's team with one Sacred Fire. If your character's a league champion and ends up being challenged by some newbie who just got their Pokémon today, yes, they'll be trounced within five minutes. Most of everything else, make it three or more. The reason why is twofold. First, it makes you lay down more description and draw out suspense so the reader can't tell who would win. Second, if you end the battle too quickly, the reader thinks you're making it too easy for one side or the other. Make them struggle through a couple of rounds, and it's a bit more interesting.
Third, always keep in mind science and logic. Yes, for whatever reason, Pokémon attacks don't kill. They don't even end fairly logically. (For example, you never see blood drawn from a Scyther's Slash.) Still, you'll want to keep in mind what would naturally happen and what wouldn't. Part of this draws in the concept Ebon mentioned about reactions. As in, if a Rock Slide crushes an opponent's leg, don't have the victim just get up and walk it off. Have them in pain, and have that injury affect the performance of the Pokémon in the rest of the battle. (For example, no, a Pikachu with a broken leg cannot perform Agility.)
But it also extends to simple scientific logic concerning what would have to happen if you used X move in certain situations. For example, if you use Flamethrower to chase down a Sentret in a field of tall, dry grass, something's going to be set on fire, and that's going to affect the battle. If you use Earthquake on the roof of Sky Pillar, that floor's going to break, and the characters are going to have an intimate relationship with gravity. And if you use Surf when there's no large body of water anywhere near the Pokémon, you'd better come up with an explanation as to how your Pokémon pulled
that one off. Point is, part of what makes a battle interesting isn't just the moves or the reactions. It's the consequences. Always keep them in mind.
If I think of anything else, I can add, but those are the three major points that I can come up with right now. Hope that helps.
