Not on My Watch
This was a mean mission! You're supposed to free the prisonsers and kill any enemies along the way. Take the Atlas.
Start out in a mech battle - you can shoot the ground out if you want so they fall in lava. Go around the road shooting any enemies you come across.
First navpoint - right to com tower. Now secure the salvage yard. "Guns pointed the wrong way". Hmmmm. Wait, it's a trick! They lured you in by claiming to have prisoners there to rescue!
Now take out the mechs. When they're dead, mission over.
MechAssault Walkthrough, Tips and Techniques