on easy / normal mode you can just use the mac cannon, it doesnt take more than 15 minutes, you dont even need to leave your base,
otherwise, send 2 scorpion tanks along the east side of the map to kill the 3 power supplies, grab the 2 in the middle, then just spam more scorpion tanks to clear the west side of the map, then when the head is moving very slowly, just move a scorpion tank back and forward through the ray to keep the head at bay whilst you kill it, only takes < 40 seconds to kill on easy / normal with a few scorpions, on legendary its the same but you will need a few more tanks to go hunting for the power nodes and the head takes about 5 minutes to kill.