There's a lot crystals of card exp and a few crystals for upgrade cards. Expirience for cards it's not a problem, but find a ways to upgrade a problem indeed. And here's a delema, buy a booster or upgrade one cards. Thats sad that theres no place for tests or tryin new builds of decks. Coz u cant upgrade them all at least to lvl 2 and try. You must predict your deck far ahead, coz even to upgrade your main deck will cost u a lot. And lot of people doesnt try other cards because they knew that they cant grade. Cant grade = waste of time. Thats sad. I think if cost for card upgrades would cut to at least half from today cost, people'll be much more happy and they can start think bout other decks or tacktiks.
The game will be more atractive and intresting if there'll be more free content. For example Achivements, if u add 30-40 more it'll be much more intresting and chalenging. Also some daily reward would be nice. And if u add exp/crystall for duel games it'll be only a good thing.
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There are more achievements, they are revealed as you achieve the first ones.