With your jetpack set to run infinitely, you should have no problem sending missiles at just about anything. Use the following setting sin Firefight, and you'll have this achievement in no time.
Choose Weapons and Damage and make the following changes.
Damage Modifier: 300%
Melee Modifier: 300%
Primary Weapon: Rocket Launcher
Secondary Weapon: DMR
Starting Equipment: Jet Pack
Infinite Ammo: Bottomless Clip (Crucial: No reloading!)
Equipment Usage: Unlimited
With unlimited equipment usage, unlimited ammo, rockets, and a high damage, you'll get this achievement within a minute!