To get this achievement you need an aircraft equipped with at least 8 Hellfire rockets. I did this on the first part of "Sharks in the Water" and it worked like a charm. You need to have at least 8 enemies on the screen at the same time.
Select your Hellfire missiles. Select a target. Now hold
. You will see an arrow or chevron appear above the target after a while. Now release
and tap it to switch to the next target. As you will see, the previous target stays marked. Now repeat this process until you have marked 8 enemies. After you have marked the last enemy you can fire the Hellfire missiles with
. All eight missiles will be fired at once and as soon as the last target is destroyed you will get the achievement.
Please note that this refers to the default button configuration.
Review 23 September 2014: Added button icon.