The rewards chest are random. A chest won at the bronze league has the same chance at flipping a Legendary Gold as a chest flipped in the Champion league. I always tell new players that basic game strategy is mele in the front (with high health if possible) magic goes anywhere and range in the back. Stick to that basic game strategy and once you get the hang of game play and all the different rules sets you can adjust.
I have just delegated you a few cards. One being the Furious Chicken in which I see you are renting. You can now cancel your rental because you have one delegated to you for free. These delegations will stay for the remainder of this season and likely longer but eventually they will be removed. Good luck on the battlefield, if you have any questions about the game ask away.
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RE: Newbie - first impressions