A domain has 12 mirror sites and all of them are indexed. As a matter of fact one of the mirror domains (44 Moz domain authority) is competing with the primary domain (62 Moz domain authority) on page 1 of SERP for a number of keywords. All mirrored sites were done via domain registrar's DNS dashboard so there is no server to do the 301s.
Is setting up each mirrored domain on new hosting server and doing one to one 301 redirects to the main domain the best bet?