The way I've played through the game, (done all levels on master now) was just by using the cards you unlock and combining some, buying some packs etc. The thing about collecting cards though is it's all down to luck, I don't have any of those more money for killing enemy's cards and it took me ages to finish my last master level. The only thing I'd suggest is on your first go through the levels I'd play them on easy then move up to normal after you've done them on easy, then master. Not really much to say, if your on iPad you could use the gems but to buy more packs of cards, pm me if you don't know what it is. Those little dudes with scythes are friendly villagers who help kill the enemy's.