Shrink wrap should work but be prepared for odd results if the shrinking mesh is close to the arms. Since the verts snap to the closest points on the original mesh sometimes they shrink to the arms instead of the torso.

Retopo will also work pretty well for roughing in the curaiss.

Any way you approach the project define the volume in lowpoly then add edge loops and subdivide as needed to smooth and sculpt. I highly recommend using mirror modeling techniques, half the work and symetry is assured.