If it's the same as Fifa, which I think it is, the general idea should be the same, as it's named the same way, you can buy packs with coins... You also earn coins just by playing regular matches either offline or online (the so called leagues - seasons mode or tournaments). These Fantasy Showdown Challenges should be the equivalent of Fifa's Team of the Week challenges, which is an offline game, where you're put against a team compiled by EA (in Fifa's case, the players that performed well over the weekend) and you get coins, if you manage to beat them (difficulty increases their number).
I love to earn my coins through trading players, but that takes a lot of work and time
