A recent Mythbusters tried the same thing - a human could stand unprotected in the focus. The shields were just too diffuse.
I hate to dispute myth busters, but if the mirror is concave, much like the mirrors used in reflector telescopes, you could set quite a few things on fire and I wouldn’t want to stand in the focus. Bronze mirrors would probably not work, but real mirrors would. At the very least a blinding light could temporarily disorient sailors of opposing ships.
A couple of years ago I watched an episode of Mythbusters where they "proved" that you cannot split one arrow with another arrow.
The next day, I watched a show on the History Channel where an expert archer did exactly what Mythbusters claimed could not be done.