It's crucial to select and use turrets that compliment defensive waves chosen. Raptors cause the most significant damage and fastest of any unit. I make stopping Raptors my first and top priority. Bombers come from same direction depending on building placement. Place flak guns in direct path to be most effective. If using rail gun turret, you must place a transformer next to it otherwise it is ineffective. Machine gun turret seems to be only one still effective without a transformer.