AMEX is a good option if possible, otherwise the short answer is to do a MR. Fly a cheap long-haul flight, you could earn 20k MQM in a couple days and for a few hundred dollars.
Yes, this costs time, and yes, you might be better just spending more for C+ or better seats, but there's no way around the MQM requirements.
Keep in mind that even if you get Platinum, you won't necessarily be able to avoid crap seat assignments if you are always buying on short notice.