Pretty good, but a little too frustrating
The game is pretty good, except it's not balanced at all. As people have said the later levels are pretty difficult. The last level is near impossible, and though I figured out how to beat it I'm not going to spend the time necessary to do so (especially since GAME DOESN'T SAVE your progress).
What people should realize is that the rocket jet and the thief are your best units. Especially when combined with the general to stop the other boss units.
***The other important thing is that babes heal the giant mecha-monsters. I hadn't realized this at first, and thought that the mechanics would heal them. The game was a lot easier after this.
To beat the 7th and 8th levels just upgrade your buildings, and after they are all upgraded use thieves to steal money and jets to kill the units that kill your thiefs. You may need to use a bomberman if the enemies are too close to your base for the jet to kill. Use the general that slows stuff whenever a boss comes.
After you have a bunch of money just get two of the $700 monster or one of each of them. Use babes to heal them and also use jets to kill remaining enemies. Levels 7 and 8 shouldn't take more than 10 minutes this way, it's pretty easy.
Unfortunately the last level, Devil's Hell is really frustrating. The strategy is essentially the same, except there are soo many of the monsters that you need to be really lucky. The main problem I have is that the $700 monsters target the opponents rocket ships and so they are really slow to advance, where as the $400 monsters advance quickly, but one of the opponents green monsters kills them completely. Therefore you have to try and get lucky and randomly guess when a green monster is going to come and time a general or not use the $400 monsters at all.
Your monsters die all the time, so you need to start with a huge amount of $$$ built up before you even start attacking (to fund the rocket jets, nuke jets, babes, and replacement mecha-monsters). I saved up $10,000; but unfortunately I ran out of money when the opponents base was at 2000 health (out of 8000). The bases heal super quickly so I would have had to start over, but if I were to try again I'd probably save up $15,000-$20,000 before I attacked; which is pretty ridiculous. It's super easy not to die though with a combination of rocket ships, thieves, and generals; so if you're patient enough you would eventually win. However I doubt it's worth the time it would take, since there's probably not even a you win screen.
It's a pretty okay game, except it's not well balanced and it's very repetitive (same strategy is best on every half-way difficult level).
To fix how boring the last level is I would do one of the following:
1) increase supply
2) make the enemy green boss unit not kill your rocket ships/ not completely kill your $400 boss unit.
3) make your $700 boss unit not target enemy rocket ships
If just one of those were true it wouldn't be so repetitive to beat the last level, although it still wouldn't be too easy.