The usual way this is done is by changing the _xscale of the enemy. If your enemy is originally facing to the right, when he's facing to the right his _xscale is 100, and when he's facing to the left his _xscale would be -100, assuming you didn't scale him at all. So you could use an if statement to test if the player is on the left or right of the enemy, and then change the _xscale of the enemy accordingly.
if(enemy._x < player._x){
enemy._xscale = 100;
}else{
enemy._xscale = -100;
}
for example...