[LazyPup]

First make sure the water pressure is turned OFF, then open the valve stem all the way. (If the valve stem is in the closed position it presses the inner end of the stem against the mixer body and puts a mechanical stress against the stem mount threads).

You should then be able to grip the stem body with the basin wrench and screw the stem out fairly easy.

If you can't get it out in that manner, you may need to enlarge the access hole and use channel locks or vise grips.

If you need to enlarge the hole, put the trim escutcheon back on the stem and trace around the trim escutcheon, then remove the esctucheon and enlarge the hole, staying about 3/8" inside your trace line to be sure the escutcheon will cover the hole when your done.