This achievement caused me some confusion as it is a bit ambiguous as to what its referring to.
The 'Missions' it refers to are the optional missions given to you by the locals in the Soviet Installation or Geothermal Valley areas of the game.
They are shown on the map as 2 downward arrows and it pulses a green circle when using
There are 8 missions in total spread evenly across the 2 areas mentioned above. Doing 3 will net you the cheevo.