Easy spot I found to do this is in Port Nixon. There is a shortcut to the medbay you find before entering the area you fight Little Johnny, when you exit from it there are 3 weak unarmored (or one armor piece) guys with big hammers to the left. I weakened them each down to about 25% health, gathered them together, which should be easy since they move very slow. Once they were bunched together I targeted the one in the middle and blasted em, achievement unlocked.
If anyone has trouble finding the spot I will make a video for you when I get a chance.
If anyone is curious where to get the missile, it is called "SSM 'Vanguard' Missile" and you get it from the Hunter you meet before fighting the boss in Gideons Rock if you tell him you will split the reward. Once you beat the boss you will find him in Seaside Court, give him his share and he will give you the missile. You will only have one, if you want more you can buy them from Rex after completing his quest chain.