Recieve fifty Guild Cards. This can be a pain, since other players seem to be loathe to send their Guild Cards, for whatever reason. What seemed to work for us is to send our Guild Cards after a successful multiplayer mission (we regularly did this while hunting Tempered Elder Dragons). Generally spirits and good will are high after such an event, and with any luck you’ll get the odd Guild Card this way. It will likely take quite a while to earn this naturally, however, so you may want to be more promiscuous with your Guild Card and just send it out every time you enter an online game.
Failing that, seek out an online thread/forum where people are boosting. Fortunately, if you have a friend you can boost this trophy fairly quickly. Just send your Guild Cards to each other, delete each other’s Guild Card, and resend them. The Guild Cards do not seem to have to be unique, just as long as you recieve fifty of them.