The thing to remember is that lightmap uvs rewrite themselves to the size of whatever your uvs are. Therefore, do what you were doing with the cliffs, but then shrink the entire cliff down. My overall suggestion is to take the largest chunks of the lightmap, seamlessly uv them, fit them together like a puzzle, and then fit all the little pieces left over in between the larger chunks. It's a total pain in the ass, but it results in fairly seamless uvs. That's what I did for Precipice.