How to easily get an expert score (or beat any level for that matter):
Fully upgrade bomb damage, 'cause your bombs are USELESS otherwise. Also try upgrading radius and price.
Upgrade your money so that you can buy a couple super towers at the start of the game.
After that, you're ready:
1. Start the level by building a couple of super towers. Don't upgrade any of them. (they're only for the small fry at the end)
2. Hold the n button on your keyboard until all the waves start coming.
3. Bring out your little bombs.
4. aim at a turn in the path, or better yet, a narrow u-turn, and wait for the creeps to start coming there. (Make sure you have a super tower after this turn!)
5. Wait until your bomb cursor is filled with creeps.
6. BOMBS AWAY! (You'll know that they're all dead when the small explosions recede.)
7. Repeat steps five and six.
You will make so much profit that you'll have enough cash to bomb the little guys INDIVIDUALLY!
 "Saucer Defense Fleet"){kind=icon}
 "mini Tower Defence (mTD)"){kind=icon}