For me it was those effing Dryads. I really have no idea how. I just know that when I saw them, it aggravated me to no end because they're annoying little pests. But I wasn't actively grinding them to get it done. It was a total surprise when I saw that their species was rare in Libra, and then the VERY next encounter, the bastard was pink/purple. Definitely a satisfying achievement to get, and not against a pushover enemy either. Makes it even more satisfying.