Staying in Diamond is great for the rewards but my cards are leveled for Gold II.

As for me, my cards are more geared for Gold leagues (highest level of my summoners are only at 5) but is battling in the Diamonds (where there is no level cap for summoners). This honestly is the main reason why I'm usually beaten. Lol

Nope. Not even close to the top 100 in the leaderboard.