First off, I'm not really sure if you understand what a short story, or “one-shot”, is. It's a self-contained story. It has the beginning, middle, and end all rolled into one piece without any “To Be Continued” at the ends or any need for prequels to get information. So your ending falls completely flat because you shouldn't have ended it with a cliff-hanger like that. The story isn't finished.
That's the main problem with this. It's very rushed. The scene where Eric and Elesa find one another should have had a lot more detail to it. The feelings that Eric has about finding the sister he never knew he had. How he might feel betrayed by his mother for never telling him, or relief that he's not alone in this world. Same with the battle against the Plasma grunts. How Eric was so focused on getting Tepig back that he didn't feel three stab wounds, or his anger that he can't find out the secret his long-lost sister has been needing to tell him.
Personally, I would have cut the first few scenes where Eric's mother was taken (killed?) by Plasma and reveal those in flashback. Start the story in Nimbasa City, telling a little of Eric's backstory so that the readers know why he's on this quest. End the story with Eric learning what the secret means and what he has to do now against Plasma.
On the mechanics side of the story, if you had the time to do so, I would have suggested a beta reader. You're missing the punctuation for dialogue, and you never started a paragraph when dialogue began. This also could have used a read-over before you submitted it. Sentences ran on when they could have been cut into two or three separate sentences. There were a few weird wordings that I had to think about what you could have meant by it. A great tip is to read your story out-loud. This way, you can hear how sentences sound when read and catch any possible mix-ups.