Hot weather is rough on Japanese Maples, try to keep them shaded and misted if possible. I can only share what has worked for me, moisture level I find is very important as dry air will cause leaf damage. Ideally a green house would work great, if you don't have a green house, a shady area that gets mist works. If your trees are planted and are well established, a deep watering should help, but if it's recently planted you may need to provide temporary shade with some sort of shade structure.

My maples are showing leaf burn too... It's unfortunate, what I did was move the potted ones into a shady area that gets morning sun but shade starting around noon and remains shaded for the rest of the day... Leaves are starting to grow back as I have removed a lot of the dried burnt leaves. Keep them watered, take time to feel the soil, sometimes the soil may look dark but is barely moist at all.