Does it even count if the person you beat forfeited because they left or timed out?
Anyhow, I've heard that legitimately beating people with Magikarp is actually possible in the Hackmons format. You just need a level 1 Magikarp with the ability Sturdy and the move Leech Seed. As long as you're going up against a poke with decent and HP, they'll knock you down to 1 HP each turn (but will not faint you, thanks to Sturdy), and then Leech Seed will refill your entire health so Study can trigger again on the next turn.
It's probably simpler to just use a Shedinja instead of a Magikarp, since it cannot have more than 1 HP ever, so you don't need to regain any for Sturdy to trigger again. But, then again, if you do that, you can't brag about handing someone's butt to them with a level 1 Magikarp.