Hero cards are earned by winning unique cards from beating specific players in Gwent. Once you have at least 3 in your deck, you can go after this trophy.
I played Vernon Roche, Ciri, and Yennefer in one round of a match and won the match. I'd recommend doing it after a round you've already won. One thing to remember is you need to have the cards come up when the match starts, so you're going to want to have more than 3 Hero cards in your deck. I had 7 and luckily 3 came up when the match began.
The trophy popped right after the match ended. Pretty easy.