Depending on the challenge that the game has set you, you might need to change the way you play. Especially when it comes to the ones that ask you to finish with a set score. Your score is based on the monsters you kill, the gems you collect, plus a small bonus for finishing a level. When you're chasing a score, try and play Tax Free whenever you can, since it'll mean you keep all the gems you collect. You can try and get through the game spending health on drawing cards, but it's really pretty difficult. Other challenges are easier to overcome. The most difficult are probably those that require you to win a set number of games in a row, since they're so heavily based on luck. All you can do in those is deal with the cards you've been given.