The mirrors are there just to bounce the lasers at you. You can take them out but the way I beat him is to dodge the lasers in that part and than shoot him with the big blue beam after you see him take the mirrors back into his arms i believe. It takes around two big beams and a couple of shots in that phase, theres two phases on the last boss. Try to keep your power shots for the second phase, it will make that part easier.