This is a fairly straight-forward achievement, but if you end up missing one, here is a guide.
Chapter 3-1 (Fog Of War)
There are 8 lightning harvesters in this level, however as soon as the level starts there are no lightning harvesters to destroy, so don't worry about any at the start of the level.
1) About one to two minutes into the level, you will notice the 1st lightning harvester on your left a little bit after the first gold pickup, immediately after you cross a bridge. Shoot this lightning harvester and at the top of the screen a prompt says, "So the Empire managed to finish their powerstations". After this quote start looking around for the lightning harvesters.
2) Directly after this first power harvester the 2nd one is on the outside of the map to your right.
3 & 4) Directly after the second lightning harvester there will be 2 seperate lightning harvesters right next to each other.
5) A little bit further on the 5th lightning harvester is right after the second large mine field.
6) After the helicopter fight on the bridge the 6th lightning harvester is off the map in a gully.
7) You will then come to a bridge with mines on it. Once again the 7th lightning harvester is off the map on the right.
8) The 8th and last lightning harvester is at the end of the level when you battle the trains. Make sure you destroy the lightning harvester before you take out the train to register all 8 for the area Fog of War.
*JUST A NOTE: When I mention some lightning harvesters are off the map, I mean the areas of the map your mech cannot walk on. Use your cannon or grenades to take these lightning harvesters out from a distance.