Going for 5* is everything to do with making combos and not so much the number of kills. Weaken large ships and finish 3-4 off within seconds of one another to pull off higher scores. Continually tell your wingmen to either follow you or send them after a target far away to ensure they do not take your targets. Use Left Trigger liberally to line up the next shot and keep those kills coming in rapid succession.
There is no difficulty modifier so do yourself a favor and make your 5* run on easy.