You will farm Thunderjaw to get Metal Shards fast. Thunderjaws will spawn at multiple locations around the center of the map, so you will go and come to those locations.
A defeated Thunderjaw drops about 150~250 Metal Shards and three of Purple modifications. Beating Thunderjaw will take 2~3 minutes if you know how to fight against it.
To defeat Thunderjaw, you will use Disc Launcher that he has on his back. You can remove it from the machine by shooting it.