It seems like a good start, but a few features could use some work. On my LG Optimus S, the enemy attacked fine, and my tank moved fine too, but the attack feature really works badly. You need to have the tank higher, and it needs a tutorial- it took me quite a while to figure out how to shoot (click and drag behind your tank, like angry birds but you're at the bottom of the screen); and while I understand the desire for having ads on your app, they're so obnoxiously in the way that most users will be far more annoyed by them than the value they add to you as a developer- plus, they block a large portion of the menu system.
That being said, the framerate is really good, and I like the way the power meter works. The powerup system is interesting- if the game gets more complex later, they could be really useful.