It is childish to think mirror deflects high power laser back to the enemy.
Yet there are numerous ways of defeating laser:
1- Thermal blooming:
Atmospheric thermal blooming
has been a major problem, still mostly unsolved and worsened if there is fog, smoke, dust, snow, rain or
purposely dispersed obscurant chemicals in the air. Essentially, a laser generates a beam of light which needs clear air, or a vacuum to work.
2- Defocusing techniques:
Laser beam has to focus on a point for a reasonable time.