42 inches is a little over a meter, so we’ll call it a meter for this exercise. Area of a circle is pi * radius^2, or pi * (0.5)^2, or 0.25 pi. Pi is roughly 3, so area is 3/4 square meters. The solar constant is 1200 W/m^2, so his “death ray” has a maximum power of 900 Watts.
The trick is the focus — 900 Watts over a square centimeter gets mighty hot.