Way too hard. So they can force player to buy boosters and coins. This game is dead of greedy now. Edit: It was not too bad before. But after recent updates, the endless challenges became unreasonable hard. Everybody in my team had been complaining about this. Some stage are impossible even with all boosters selected. I have spent some money on this game, but I am not looking for frustration. Please be reasonable and try the stages yourself before releasing them to players.