I use a 4" side grinder with diamond blade or a rotozip most of the time. If the wall has wooden lath I cut the plaster then change blades in the roto zip and cut the wood. Running the shop vac helps keep the mess down some. If I need to do a clean job (no dust)I use a hand saw, slow but less messy. A sawzall tends to grab and pull the plaster loose and make cracks. Rod