You'll start this mission with a machinegun, but will be up against snipers and other long range targets. Take the time to move to the right and grab a rifle with iron sights from the fallen ISA troops. This will make it a lot easier in taking down the Helghast that have a decent range advantage.
Continue ahead and meet up with Narville. After a short scene you'll be buddied up with two random ISA troops. Arm yourself with both an assault and combat rifle; a shotgun pistol will also be nice.
You can't really rely on the assistance of the ISA, as the Helghast will ambush with flame throwers. Let your allies act as bait and kill off the flame troops with your rifle. However, the enemy will be in the upper level above the trenches, making it unlikely that you'll be able to grab the flame throwers for yourself. Instead arm yourself with a close range weapon, such as a shotgun and continue to march forward. A prone Helghast will be just around the corner, but a good shotgun hit or brutal melee will make quick work of him.
You'll now go toe to toe with the MAWLER, the huge beast of a machine from back in Chapter 3. Head into the shack and grab a mortar launcher. Remember, just like in Icy Incursion you'll want to attack out of sights to conserve ammo. Start by killing the Helghast gunner stationed in the MAWLER's knee caps. Immediately afterwards take refugee behind the giant battery stations. Be warned, if these batteries are damaged too much they will let off a deadly electric shock.
After the MAWLER uses its main gun it will need to cool off its generators. Use this time to target its weak points and take down a brunt of its defenses. Eventually the MAWLER will ignore the Raiders and go straight for you. Avoid sitting in the shacks as the enemy will specifically target that area. Instead stick by the batteries and only move when they let off too much of a charge.
Helghast drop ships will appear to unload ground troops and the ISA will have little in the way of resistance. Kill off only the enemies that come your way and stock up on more rockets. As soon as the MAWLER use its main weapon and cools down, drop everything and target its exposed weak point to take it down.
Watch yet another scene and the MAWLER will start back up. For now you'll be moving under the beast and taking on more Helghast ground troops. Your ISA allies won't be much help here, as the enemies are packing powerful weapons and are using mounted turrets with shields. Use what little distraction the ISA provides and score some headshots with your rifle. You'll want to focus on the turrets as they are the most deadly obstacle on the field. After clearing those out, cautiously move up the stairs and shotgun the remaining Helghast on the lift.
You'll be picked up by the Raiders and will once again be a gunner on one of their crafts. Concentrate on taking down the MAWLER's anti-air guns at all cost. You won't have the luxury of cover, so your only means of protect is a swift offense. Even when your goal switches to taking down the MAWLER's power core shoot out the remaining AA guns first. After its offenses are finally gone, put the final bullet to end the MAWLER and this chapter.