There are several factors that could make this extremely difficult to accomplish. The asteroid may be tumbling in a random fashion at a high rotational velocity. Even if one could manage to land, using thrusters effectively on a tumbling object is problematic. The surface of the asteroid might be extremely irregular and rocky making the landing treacherous. Furthermore, the composition and properties of the asteroid are likely not well known which makes it difficult to design anchors to plant into the surface.
Another idea I read about is to launch, rendezvous, and set up a large reflective mirror that concentrates sunlight on the asteroid which would heat it up and cause out gassing which would deflect the orbit ever so much. The mirror could be made out of light weight mylar film. If it's tumbling at a fast rate I don't know what you'd do.