I’d go with a sawed-off 12-gauge myself...
A shotgun is the wrong weapon, the ammunition is large and heavy, target aqusition is slow and labor intensive, reloading, and the wear and tear of the shooting would consume time and wear you out.
AR-15s are ideal, they are built to quickly engage multiple targets, have 40 round magazines, and are meant to reload in 3 or 4 seconds.
Remember, the object is to pop their heads, not ‘blow them away’