This is the hardest achievement in the game due to the randomness that officers appear. Once in a while after a mission, you will notice someone in your town that isn’t normally there. Go talk to them and they will either present you with a card, upgrade your skills by one, or say something random. It took me hours upon hours to gain my last card. I will provide my theory in hopes that it will help you gain them all.
It is my belief that the cards unlock while a certain faction is in a certain chapter completing missions in a specific campaign. Yeah, a lot, I know. So once you complete every mission and do everything in the game, pull up a word document. If you have a good memory, then do it by memory. Here is an example. Start on Chapter 1 of Wei, with a Shu Officer. Play all missions and hit start during them so you can return to city, ending in defeat. Do this until someone appears. If they give you a point into your weapon or say something random, go on to Chapter 2. If you do get a card, keep doing this until someone appears that doesn’t give you one. Do this with every faction on every chapter. Wei on Wu, Wei on Wei, Wei on Shu, Wu on Wu, Wu on Wei, Wu on Shu, Shu on Wu, Shu on Wei, Shu on Shu. You should eventually find your missing cards.
Let’s make sure you understand. Give up on missions with every faction on every chapter in every campaign until someone shows up. If they don’t give you a card, move on the next chapter. Once you get to Chapter 6, move on to a faction you haven’t used yet. If you used the same character from the start on every chapter in every campaign, and you still don’t have that character’s card, it’s because you will not meet yourself in town (though you will in battle from time to time). I have no real way to double check this, except after 10-15 hours of straight quitting missions, I started changing my factions and saw five within two hours.