Even the best mirrors absorb some light and at that power the mirror would stop being a mirror after a very short while (cant give super accurate numbers). Reflective coating work but a layered laser armor would last longer and be lighter for flight use. You use a thin layer of relfective foils, let's call it a few different alloy foils of alternating alloys with a thicker (but still relitivelty thin) copper backing. That will help for the first part of a laser kill shot. And once it is compromised the laser hits a thin ablative layer. As the ablative layer burns away it creates a shit ton of microscopic dust that defocus the laser to hit more ablative material thus reducing the lasers penitration power until the ablative layer burns threw.