On Fri, 02 Dec 2016 20:04:21 -0700, Charles Bishop
wrote:

I would use a reciprocating saw with a short, bimetal, metal cutting
blade.

Won't this shake the wall, perhaps causing additional damage?

Quite possibly. The reason I suggested a 4" angle grinder with a
diamond blade. A plunge cut with the blade is much easier, less
damage.