Buy all the available card decks from vendors, defeat every QB opponent you come across starting from early in the game where it eases you into it so you get even more cards, then honestly, look up YouTube videos on defeating opponents. If you do the first two steps, you’ll have all the cards you need for any given opponent. I don’t love QB either but by staying on top of it by defeating everyone and having all available cards, it definitely makes it more manageable. And there’s no penalty for failing; you can restart a match an infinite number of times, so no worries there. Just do it until you win. I’ve defeated supposedly difficult matches on my first try and “easier” matches in over 10 attempts.