The hardest acheivement that I've gone for and succeeded at in recent memory was the Medal Seeker achievement in Dead Block. It required four gold medals on every level, and the only way you can get gold is by playing on hard. The medals were for destroying and searching through all the objects (of which there were MANY), killing a certain number of zombies per level, and not dying. The hardest medal to get was for not dying, and it was pretty much impossible to get all four medals at once, so I would break it up.
