If 15 degrees NU still results in a negative VS, all you can do to avoid ground contact, is to pull up more. Up to the stick-shaker, if necessary.
I think you read what I wrote too fast... But I will elaborate:
Wind sheer escape maneuver does NOT call for raising pitch to stick shaker, but as you wrote 15 deg ANU. Even if this results in negative V/S, you hold 15 deg (or what ever value applicable to you aircraft), as long as ground contact is NOT a factor.
If ground contact becomes a factor, you are no longer flying a "wind sheer escape maneuver", and the wind sheer is no longer the priority. Because it's obviously far worse to risk a crash. Hence, the wind sheer escape transitions over to "terrain avoidance maneuver", which calls for raising the nose to intermittent stick shaker.
However, in e.g. 1500 feet during approach or takeoff, and getting into wind sheer, it makes no sense what so ever to raise the nose to stick shaker and risk going into a stall. In addition to not making sense, it is not correct procedure either.