well it might just be that way because frogs have naturally slimy skin that fights diseases, can poison enemies, and can withstand certain acid. So in that sense, the frog would own the slug, and the slug can acid the snake to death. And the snake can't really do anything to the slug as we saw it just break away when it was squeezed. This is all assuming the slug can only do acid attacks and that the snake is weak against it.