Which Cards Should I Upgrade?
As work your way through the Arenas you will be able to unlock new players. You might be tempted to upgrade each and every player you can when the opportunity arises, but try your best to prioritise advancements that fit your tactics and players you want to take into the Arena.
There is no use in upgrading a Good Ball Playing Defender that will do nothing but sit in your inventory for eternity and never be used. Try and single out the players you use the most and level those up the best you can. That way you will be entering the Arena with high level cards and ones that you have become familiar with.