super scarab

I took Forge in his warthog right up the centre of the map right up to the front of the scarab. Then moved from left to right of it's head so I didnt get shot by it. I completed this mission in about 5-10 mins doing this. Also using the MAC cannon on the scarab speeds this up as well.

The way you can do it is really easy, take Sargeant Forge in the warthog along with the Scorpion tanks that you got from the beginning of the mission, don't even bother with the power nodes that are along the way because you can get the achievement for not destroying any of them. Destroy any of the Locust tanks you find around the map, I believe there is 12 or 15 of them in the level.

Follow all the way up the center of the map and place a rendezvous point behind the barrier on the far right side so that you constantly get the vehicles to the same place so they don't get obliterated by the Scarab head when it turns the corner. Press up on the dpad when you're in place and fire the MAC cannons on the Scarab whenever the head is facing your position, rush out and bombard the mechanized monster as it's head is facing the other direction and quickly jump back into cover as it comes around again and fire MAC rounds again. Repeat the process until the Scarab is defeated and you'll get the achievement as well.