If it is plaster lathe, cut the plaster with a rotory saw. THen cut the slats of wood with a cordless sawzaw. I didn't see that it was plaster. I freakout whenever we have to cut into plaster. If the plaster has come loose from the slats, the whole wall could come down.
I would definetly suggest getting some help. If it is drywall you have a different story.
