The fastest way to kill them is to knock 1 down with the buster and another one with a charged shot. Than you kill the 1st with buster and an air combo and the other one with another charged shot and buster/aircombo. You can also use the Deviltrigger if you have enough bars. Faust with grab and charged shot to get rid of shield and then use Deviltrigger with busters till he flies away and then finish with another charged shot (prevents them from using their special attack and saves your life later waves).
But i guess there must be others places where you loose a lot of time. Do you get the Blitz down fast enough or the bosses? How much time do you have after the 1st boss? Maybe you can tell something more like your strategy so far because i doubt that you loose about 20 minutes only at mass mephistos