A few ideas to make the game harder:
1. Change number of enemies spawning. Add cards with number 4 to the deck for example, or always add 1 to the card drawn.
2. Reduce the number of heroic points, or make them roll over to the next mission so you have to choose to use them now and run the risk to really need them later.
3. Make the enemies move faster (5 instead of 4) so they do get a chance to get close. When they are unrevealed, they actually move the equivalent of 6.
Another comment: I found the melee option to be so much worse than the ranged option that I barely used the Brawler's ability. I preferred for him to shoot, than risk getting hurt during melee.