To get this achievement simply finish Escort type missions (orange dots on the global map). You will get two soldiers for each mission, so it will be a minimum of 13 missions with 100% completion to get the achievement. The counter goes through all of your save files, so if you have problems with difficulty at later levels, just start a new game and save soldiers there.
The general tactics is to keep soldiers behind, and run straight to the map exit. Leave those intels behind at least on the later levels, as there is a minimum profit from getting all of them, but going through all the corners of a map will significantly increace the chance of your soldiers be killed. Soldiers are pretty tough and have a good firepower but they cannot heal themself.