CAUTION Used 2 medals to unlock him first or if you run out of medals you have to use real money
What I did was play the game upto operation 2 level 4, then used that mission to farm the 3 million points (as posted in War Bonds by Cam is Kinected excellent guide) which only takes a couple of hours. After that I selected the machine gunner for every mission that was left in the game. He costs 2 medals to unlock and 50,000 credits each mission.
He is very handy and I kept using him after I unlocked the achievement (which only took 3 missions) since I had loads of credits and he cuts down the enemy quickly.
If you need to farm this achievement then selecting 3-7 mission is ideal as that level has tons of enemies and as the machine gunner shoots faster will nearly always get the kill.