Theres not enough attack cards, especially for how many enemies there are, even with upgrades. Please let there be more?
I'm thinking about adding a lower difficulty level in the next update, where the pressure on attack will be a bit less (we'll see if I can fit it in).
In the meantime, it always helps to look for other sources of attack - especially focusing on getting stronger card combos can give you a lot of attack (and defense).