i think that [with the targeting] the shots slow down the enemy so that other enemys can over take the one being shot, though the turrets don't follow the leader.
allso to get past the cat you click on the apple to make the alien eat it then the cat runs over to the spider then you click the door handle to fall down then you click the hole behind the cat so it turns around then click the shadow of the tree branch directly above you then click hole again then click the taps in the order their in from botom to top. finaly click on the cats ass to give him anal and kill it