Set between 4th to 6th century silvered glass mirror has been invented and were seen in battles all over Europe. How do the medieval militants use the mirror to turn the tides of war? (Archimedes's heat ray?)
A few possibilities for you, that are unfortunately unconfirmed.
1: Signalling. You can reflect light off of a mirror into someone's eyes to create a 'flash' of light that can be seen from a substantial distance. Used correctly, this could carry simple messages across great distances. As an added bonus, anyone not caught by the 'beam' of light coming off the mirror would not be able to see that the message was being sent at all. The messages would be very simple, as Morse code had not yet been invented. There's also the restriction that it only works when the sun is out.
2: Observation. It is a lot harder to see a small piece of reflective material stuck around a corner or the side of a tree than it is to see a person sticking their head out. Even the modern military uses reflective materials to look around corners without exposing themselves.
3: As mentioned above, you could use mirrors to temporarily dazzle or distract enemies. As marching into the sun is a bad idea, you'd want to have mirrors that could be angled to catch the sun. In open combat, this would be hard...but distracting a guard to make him look towards the mirrors, and away from where he's supposed to be looking would be much more doable.
The unfortunate truth is that it is very difficult to weaponize a mirror...especially ones as primitive as what existed in the middle ages.
If you put mirrors on your soldier's shields, and face them towards the sun, they can blind their enemies with the reflected sunlight (for a great example of the effectiveness of this tactic, see the battle of Helm's Deep in the film adaptation of Lord of the Rings:The Two Towers). There's not much that can be done to combat this tactic, as sunglasses won't be invented for another few centuries, and you sort of have to be looking at your enemy to fight them.
The only flaw to this strategy, and I think it's one Sun Tzu warns about, is that you should never point your army at the sun. Worst case scenario, you'll fight a battle where no one is able to see. But if you get the angle right, the sun should be high enough to not get in your eyes, but reflect into the enemy's.
I can only think of a few reasons why this wasn't actually done in the past: 1)- mirrors were too expensive, and 2)- soldiers were too dirty. 2 can be solved by holding some water in reserve, and washing the shields right before battle, but 1 is really up to you; get as much mirror as you think the army can afford, and divide it up among your front line.