This achievement can be easily obtained in the training missions.
Use the first building site to set up a barracks (walk on top of it and press X).
Then wait until a squad of marines (a squad consists of 4 marines) is trained, when that is the case deploy/assign them to another building site using LT. (this might take a little effort because you MUST point them exactly onto the building site.
Leave the marines standing there with 4 on that tile, walk onto the tile yourself, press either X or Y to build a barracks or turret. As soon as the building is created/dropped it will whipe out the squad of marines. You will also hear "Squad Frag".
Once that is done, finish the mission (win or lose doesn't matter). When the mission is over the achievement will unlock and you have just gained another 25G.
If anyone has any comments or suggestions for improval, let me know and I'll update the guide. ;)