I made it to level 96 on stage 6, when I came to the conclusion that the game is impossible to beat without any cheats. I then talked to various gamer boards and the consensus was unanimous. Part of the problem is that the last two fliers have something like 9999999999999999999 hit points so that is two life points automatically right there. The other problem is that there is no balance of ground to air turrets to properly win.
The game is beatable with cheats however, I've done it. The key is to just focus on the air units and not worry about the ground units by keeping them in the maze-trap. I did that by making a giant "C" and letting them go up one side of the "C" then closing it off to force them back down to the bottom part of the "C". As for the air unit. Air-Missle turrets are not enough. You have to have freeze turrets to freeze the air units down so they get hit enough. But you have to make sure you put the freeze turrets far enough away so they don't focus on the ground units instead. Then you have to put the turrets in a bunch (right next to each other). Don't make them part of the maze where they can be spread out. Basically you will make a cube where the top and left hand sides are all freeze turrets and the right and bottom and all of the inside is made of missle turrets.
The worst part is, when you win, all it says is "congratulations" instead of "game over". Pretty disappointing to say the least.