That is impossible. Battle videos are real-time RNG seeds and move data being broadcast in a specific order to simulate what happened in the battle, so they're not real video files and it's not really possible to tell what the videos even are. A capture card is required. Get ready to spend $100+ for a decent one.
I am no expert on them myself, but these are the things that Twitch streamers use. From what I can tell, Loopy and Katsukity are good brands, but they also appear to be fairly high-end.