I would advice farming Silver Arena for shards instead of Bronze. Why? There is higher possiblity of getting them from normal spins [as many Silver Arena's decks contain these]. Plus you have to win only 5, not 6 games 
[oh, how annoying it was for me to lose the last fight in Bronze. I always thought to myself: see, Sil, at Silver Arena you would have special spin now 
]. I have already won 1 SoSe, 2 SoV, 1 SoD and 1 SoG there and I started serious farming...well, like few hours ago, lol.
Wishing you luck with spins. - Sil.
silver arena often has more "annoying" decks, and stoppers (the term I am using to call decks that win over you, and stop you from getting a rare spin)
also, each match in silver come out to take longer than bronze matches on average
