I have no experience with rock nem removal but have removed several bubbles. It is extremely difficult to remove the foot without tearing or damaging. The problem of course is that the nem retracts into rock crevices. I have found a "butter knife," to be the most useful tool. No the actual butter knife but the one with the serrated side because it's thinner. Of course avoid using the serrated side as that will guarantee damage to the foot. You simply must be slow and methodical, gently sliding the knife edge under the foot and lifting. For the bubbles you'll have to do this process carefully about 450 or so times. Keep in my, never done this on a rock nem!
If you begin and cannot slide anything under the foot you could try "encouraging," the rock nem to move to another rock. This method probably isn't feasible for post people. If you have room, you could shade the nem or in some way make it want to move. I've never tried this method, but it sounds good in my head!