Everyone. Literally. All the pieces beat the Spy in Stratego. The Spy is ranked "1", and so is defeated by every other unit. Even the other Spy, also ranked "1", will cause both troops to be removed from the game. The Spy's only purpose is to hunt down the Marshal (10). If the Spy attacks the Marshal, the game's over - the Spy has won it! You could also use it as a bluff piece to buy yourself some time, although that's rarely a good use of resources.