If we're basing it off anime and manga, then I suppose it could be anybody's game, haha. In terms of anime, even an un-evolved Pokémon, such as Piplup, could take down something big and muscular like Machamp. It's been proven plenty of times before, so I don't believe much people would be able to disagree on it. Same goes for manga, if I'm not mistaken.
As far as games go, Scyther could take down Heracross, but I think that latter will always have an upper-hand. Its offensive stats are better overall, and it has access to a variety of moves that could take former down in just a single hit, such as Stone Edge (as mentioned above). Not only that, but if it has Guts ability and happens to be holding some item like Flame Orb, then its Attack stat would be raised dramatically, making it even easier for the Pokémon to take Scyther down.