"stop the drilling device or all is lost"
2 doors ahead, tunnel behind. Close tunnel to prevent reinforcements.
Take out tower on right, take out mech-friend. Now blow up inner buildings from outside the door. Eventually go in and take out any remaining mechs and tanks left over.
Now turn right and take out the mech depot and repair yards from far away so they don't come after you. Ton of health lying around.
pretty much at max everything now and there's still more upgrades lying around.
Go around corner in road and get a few tanks, then an Atlas and gun turrets. When the Atlas is dead, go back and replenish starting with the things furthest away from you (so later the stuff you need is right nearby).
Now you're at a lava field. Stay back and take out the mechs that come at you. You can't fly so take the shortest path across the lava areas. Go under the archway. Get the next Mech to trap himself against the left lava and he's easy to kill. Now you're under attack from several mechs. Lure them through the lava when possible and try to blow up ones that are near other ones. You should be able to get back to max of everything from their corpses once you reach the big wall.
MechAssault Walkthrough, Tips and Techniques