Mark the location of your cut, using a chalk snap line. Have a partner hold the loose end of the snap line while you extend it to the end point of the line. Pull the snap line taut, pick it up from the wall's surface, let go and let it "snap" back to leave a chalk line marked on the wall.
Score across the length of line, using a masonry chisel and masonry hammer. Hold the chisel's edge directly on top of the line with one hand. Use your other hand to swing a masonry hammer and strike the chisel's butt. Score along the entire line to a depth of 1/8 inch.
Set the circular saw's depth to ¼ inch. Activate and lower the saw onto the scored line. Push the saw across the length of the line, using the scored line as a guide. Deactivate and remove the saw from the wall. Adjust the saw's depth to ½ inch, an increase of ¼ inch.
Activate and lower the saw onto the scored line. Make another pass through the line. Deactivate and remove the saw from the wall. Increase the saw's depth setting by another 1/4 inch and, again, cut another pass through the line.
Make successively deeper runs through the cut area, increasing by increments of ¼ inch, until sparks fly from the wall--sparks indicate that you have reached the metal lath beneath the stucco.
Remove the masonry blade from the circular saw and attach a metal cut-off blade. Do not alter the saw's depth setting. Activate and lower the blade into the cut area and make a pass to cut through the lath.
Remove the metal cut-off blade from the circular saw and replace with a wood-cutting blade. Make the final passes through the wall's wooden sheathing to complete the cut.