Actionscript For Duplicated Movieclips

so heres my problem, im working on a game where duplicated enemies come scrolling in from the side of the screen. and ive made a turret thats automatically supposed to aim at a random enemy.

heres my code that i put in the turret:

onClipEvent (enterFrame) {
myRadians = Math.atan2(_root.enemy1._y-this._y, _root.enemy1._x-this._x);
myDegrees = Math.round((myRadians*180/Math.PI));
this._rotation = myDegrees+90;
this.setDepths
}

since its _root.enemy1 it only aims at the enemy1. and if i change it to enemy2 it aims at another enemy. but what i want is for it to aim at a random enemy? how do i do that? please PLEASE PLEASE help me. :pac: