Just to expand on that point.
You'll find this a lot easier by using hit tests. You'll have to tell us what version of ActionScript you wish to use in order to get the best help.
But if you tween (be it by motion or by scripting) some enemy MovieClips. By then testing, using an enterframe function, you could then implement a hit test function to see if the mouse has hit one of the enemies.
Once it has, you could then use a flag, or boolean to trigger an event that occurs, once the mouse does come into contact with an enemy. Such as a loosing screen, or a lose of a life.
Sorry if thats really confusing. ;)