I realy like this Game but I'm playing Mission 7 for the 87986876 time and always failing to defend against 40 Swarmers Waves. They are fast, have moderate health, are ultra fast and make good damage. The only way to get them is with 2 or 3 missile launchers, but you wont have enough money and time to build them up where you need them.
On the other Hand there are mission where you fall asleep because they are too easy. I dont know but i think you should adjust the difficulty more to a moderate level which is not boring but challenging without being frustrating to much.
I will try to beat mission 7 again with hard difficulty -_-