I don't know anything about AO .45 grip mounting procedures.
However, whenever you have a tight screw that needs to be loosened without burring it up when the screwdriver cams out of the slot, here is what I do:
Clamp whatever device the screw is in (the "substrate") in a rubber-jawed drill press vise.
Find a screwdriver bit that just exactly fits the slot. Doesn't hurt if it needs a wee bit of persuasion to get seated in the slot (but it must be fully seated).
Chuck the bit in a drill press. Be sure the cord is unplugged.
Insert a rod into the hole in the chuck that is used for tightening the chuck, bear down on the bit with the rack, and work the bit (via the rod) until the screw starts backing out.
Pressure on the bit via the rack allows you to put far more torque on the bit without camming than you could ever do manually.