You can if you know the enemy must pass through a specific area, like when they sail into a harbor. Then you just aim at where they have to pass through... I don't care what Adam & Jamie said, the sun's putting out lotsa power, and it can be focused. The question is whether they, in ancient times, had smooth enough, clear (efficient) enough mirrors, or enough of them to overcome the lack of efficiency. If you had reflectors that only operated at 50% efficiency, on a bright sunny day, and you could aim them reliably at a point, and had say, a square mile of them, you could melt ROCK with it, I would think. I don't know the amount of power the sun puts out on the Earth in Greece, or what time of year... but I'm sure it could be figured out by trial and error, if you had to.
OTOH... as soon as word got out, the enemy would simply attack only on cloudy days, or at night. HOWEVER, what if they could re-aim them at a big bon-fire? Maybe history has it wrong, and the point of the mirrors was to BLIND, not burn. In such a case, even a big bonfire could work. If the sailors on the enemy boats were unable to see, they could then blast the boats with flaming pitch from catapults, or whatever, or just hose them down with something flammable using high-pressure monitors, or maybe just flood the area through which they had to sail with oil, and set THAT on fire with the sun. Maybe they did that.
OTO, OH, maybe they didn't use the sun at all. What if they used a bonfire, and the mirrors were for focusing not visible light, (although they doubtless used that for aiming) but instead was for directing IR? They didn't have to know what IR was to know fire was HOT!!!