22 Mar 2014 04 Aug 2014
While Kuraimari13 solution is perfectly fine when you want to get this achievement quick. I'd say just go for all gold medals and missions specific ops. When you have those you'll be very close too, or even several thousands worth of Xp over the requested 25,000.
So why bother getting it fast when you get it while playing natural?
And if you come short on your 25,000 after completing everything, then Kuraimari13 still has the best solution after all.
EDIT: So while only having half of the mission specific ops and most of the gold medals, I got the achievement. Also the rewards you get from the specific ops count towards the 25K