[FLASH CS3, AS 2.0] I need help with my game project

Hey I need great help with this actionscript game I’m making for school. I’ve got some half-assed version with very flawed scripting, so if anyone could spare some time to give me a few run downs on where I need to head to and things I’m doing wrong, I would appreciate it.

My main problems are:

-Game starts lagging halfway through the game
-I have no idea how to make multiple instances of the same enemy without recoding individual hittest scripts for each enemy and copy+pasting everything, and renaming their instance locations.
-How do I adjust the health levels BEFORE the gameplay actually starts - i.e. creating difficulty levels.
-How do I save score throughout the entire gaming period, and have incremental values of overall score (accumulated through all the games I’ve played)
-A tonne of other little problems

I’ll attach the file below, so please give me some tips, advice, or ways to fix my piece of crap. Also if anyone’s awesome enough to give me very in-depth help, that would be sick.

THIS IS MY FILE >>> http://www.2shared.com/file/5755870/dab99805/test1_cs3_version5.html

Thank you super code fighters