This only requires being neutral with the Red Devils faction. Go around to Outsider stations and pick up Red Devil missions and do those until you are neutral with them. These missions will generally require you to do something to negatively affect your standing with the militia so sprinkle militia missions in there being careful not to take missions where you fight Red Devils. This takes a lot longer than it seems. Gaining rep is a very slow process and there is no way to gain Red Devil rep besides doing their missions. I sat on the border between Red and Grey for probably 20 missions before this popped.