How to Get the "My Precious" achievement in StarCraft 2 (Colonist mission Safe Haven)

One achievement for you to earn in Starcraft 2: Wings of Liberty is the "My Precious" achievement, and you can get this in the Colonist mission of Save Haven. In order to obtain the achievement, you must successfully rescue two Colonist bases from the Protoss attack on HARD difficulty, i.e., destroy the Purifier while at least 2 Colony bases are left un-purified!