My only complaint with the game is that in the 4th set of missions, enemies have a knack of spawning in the bottom 1/4 of the map, this makes it extremely difficult to kill them without being hit (as weapons can't attack sideways)
Firstly, not enough space, secondly, utter BS how you get forced to the edge of the map to dodge a plane, then just have to sit and take it as a plane comes from the edge of the screen to destroy all your flock....otherwise, it was enjoyable.
I have an idea that might help with the dearth of energy and stamina, maybe after winning a battle, or completing a mission, you get 10% of the energy or stamina used in that mission back. I know it's not much, but for example, if you start from full stamina and win all 10 battles, you get a total of free 10 stamina to play again, it's not much, but a couple few extra battles/missions could help in finishing a level or getting that last bit to buy a new reward card or pack.
Hint: having trouble getting gold? Mass rush on Dragon's Lair (level 10), and usually you can claim victory before the dragon comes out! This makes for an easy 10k each run.
For those of you complaining that the creatures are too hard/impossible:
I defeated the game without the spread upgrade. Frankly, I did not know there was one until I saw the achievements. I admit the final boss gets a bit frustrating, but there is a platform that you can stand on ~ 60% of the time and be safe while shooting the boss. You have to find out which platform that is. After finding the spread upgrade, it becomes way too easy. So for those of you that say the game was too easy, try it without the spread upgrade. :D The story, although done before in other games (in essence), is still good. And I like the new ending. :D Oh, and the only time I died was on the final boss. Tip: health low? farm until a health bar comes up.