You must be at least a Rank 20 in order to receive Helicopter backup. Upon reaching this rank, Merryweather will contact you and inform you that they can provide backup in the form of air-support units to your location. Once you have received this information, open your in-game cellphone and select "Merryweather" in your Contacts. Once the call goes through, you will see a tab with their services pop up on the screen. If you have enough money, select "Helicopter Backup", which costs $5,000, and the achievement will unlock immediately.