What is a strategy to earn the best defense achievement in StarCraft 2?
I have been attempting to earn the Best Defense achievement in StarCraft 2. The Best Defense is an achievement in the third mission of the Wings of Liberty campaign. The goal of the mission is merely to survive 20 minutes, to await evacuation. However, the achievement is to destroy 4 Zerg hatcheries during those 20 minutes. Thus far my strategies to accomplish this goal have failed. Has anyone devised a winning strategy to earn the achievement?