If you want to go for score, I'd suggest using fully upped Shrieker rush deck and grind AI3. You prectically can't lose a match this way, and upped Shrieker rush is really fast, so your score will steadily go up - unlike grinding FGs, where even best decks out there autoquit or lose 40% matches (still, FGs are good for grinding gold, as they give upped cards).