Seems you have two options if I understand that you have a leaking pipe in the wall leading up to the shower.
1) tear into it from the shower side.
2) tear into it from the other side.
Option 2 may be easier if you have tile or some type of surround that would be hard to patch. If the other side has drywall or something easier to patch, use that to get access. There is no good answer to your problem.