I have videos on Youtube for beating Hard levels on Hardcore without losing a life. You can make things easier by switching out of hardcore mode (when you launch geoDefense, go to Help & Options and touch "Hardcore Mode" to switch to "Novice Mode"
You can see which levels you've completed by looking for the green check mark. You can see the levels you beat without losing a life by looking for the yellow star.
It's hard to see some of the details in the videos, so if you have any questions about what I did, please let me know.
Finally, there was an update to the game about 1 week after it released, and I believe that made some levels easier. IIRC, the target area for the missile launcher got bigger, which really helped on some levels.
Link to the playlist: http://www.youtube.com/playlist?p=PL1214895BA93C924Fhttp://www.youtube.com/user/SwamiMacGyver
I've included the video for The Eye level. Go to the playlist if you want them all. If you really want me to add all 14 youtube links, let me know :-)