Im re-working a tile shower set on a concrete slab, and have removed all tile and mortar down to the slab, ready to start rebuilding. However, I cant get the bottom section of the brass drain fitting off so I can put the new waterproof membrane under it. Its about 2-5/16 hex nut – Ive been putting every penetrating oil I can find onto it, trying to grasp the edge of the nut with a pipe wrench, but cant bet it to budge.Any suggestions? (Im not at all interested in cracking the slab to replace the entire fitting!)