Just play though the campaign on easy and go for this on your first playthrough if you let anyone through your green health bar in the upper left corner will diminish if this happens or if you loose the level immediately restart the level. look at my guide for classic easy victory achievement for help. if you finish the entire campaign you can go back and use level select to restart levels you had trouble with but by the time you finish the game you get used to having the upper level units and forget the good strategies with the lower level units so just do it right away and every time you fail tweak your method a little if your having trouble with the flying units set up the anti-air flying units, you'll figure it out. Good Luck!!