It seems no one has mentioned that USdbaAA has eliminated the Challenge feature (gold as well as plat) so you need to qualify the old-fashioned way. You can either fly a lot of segments (people doing this on their own dime look for cheap, short flights with one or more intermediate stops that turn around and take them home the same day) or amass 50,000 Elite-Qualifying-Miles. The arithmetic for turning flight miles into EQMs has gotten more complicated and generally favors flights marketed by 'AA' over OW competitors (although the latter may sometimes be more cost-effective).

As recently as last year (for 2016 eligibility) 'AA' has offered late-season buy-ups to people who were short of a threshold, starting around USD1000. No guarantees it'll happen.