"It was soundly debunked on Mythbusters last season."

I wouldn't rely on Mythbusters for science. Generally they try to reproduce one version of a myth, fail, and announce it "busted". In this case they were unable to control their focus well enough. More people, more training, and more time and experimentation could have helped. Mythbusters tend to be very restricted on time - they probably spent 3 days on this when the ancients would have spent years; and Mythbusters had about 20 staff where the ancients had armies of men to practice with. Scale matters.

There is more on this at the slashdot site. Interestingly, one of the problems is for each soldier to figure out which spot of light corresponds to his mirror/shield. Some kind of aiming mechanism, such as is used by a modern one-handed signalling mirror, would have helped. If they had such, it's been lost.

And unless you have access to a laser you will not be able to concentrate the light sufficiently to reach ignition temps on a wooden ship at distance. The Mythbusters fixed array consisted of >100 mirrors all aimed to a single point at a set distance. This configuaration was decided upon after having the staff try, and fail, to accurately aim the mirrors by hand. The tar sealed target was tethered at the requisite distance and no ignition was achieved. To have 100 troops, even well trained troops, hold aim on a single point on a moving target long enough to achieve ignition is not believable. Remember, this is not tinder being lit by the concentrated beam of a Fresnel lense at 6 inches. You're talking about large dimension lumber at hundreds of yards. If you choose to believe in this fantasy then I will leave you to it.
Ed

Mostly I'm warning against putting your trust in Mythbusters. If you have other reasons to disbelieve, that'#s fine. For what it's worth, you don't need a laser. Light from the sun is already as near parallel as makes no difference. The size of the mirror makes a difference. What the mirror is made from makes a difference. The number of mirrors makes a difference. The soldiers would have had more time for training, and more motivation, than Mythbuster's staff. I agree they might have used a better target than timber; the boat's sails, perhaps, or rope rigging.