Destroy First Anti-Air Wraith

The first portion of this level couldn't be easier; it involves opening a series of doors and following the Warthog as it takes down most of the enemy threats.

After disembarking from the Warthog, open the door ahead via a panel on the right wall. Walk inside while your teammates follow in a hog up to a second door, only the control panel is on the other side. Either take the nearby staircase or use the wooden box to leap to a walkway on the left, hang a right and quickly interact with the switch to open the door (ignore the enemies below for now). Now drop through the slit and hang back by the door while the Warthog takes care of business.

Once clear, open another Warthog blocking door and follow behind it as it devours the hallway. Open yet another door at the end of the hall (leading to a large field) and take one of the two ghosts parked in this room.

An anti-aircraft wraith is stationed near the field's center - you must destroy this to complete your first objective. Thankfully, there just so happens to be a missile-pod nearby which was designed for just this occasion. Ride the Ghost along the walkway, mowing over any enemies in your path. When you reach the bridge that leads to a center building, hang back and wait for the hovering Phantom to depart (use the boxes to avoid its plasma blasts). Once the Phantom is gone, proceed forward and use the Ghost to shoot the enemies inside and on top.

With the building cleared, run to the top to find the Missile-Pod lying on the ground, along the edge overlooking the bridge. Use it to quickly fire four rockets at the anti-air Wraith to take it down. You should have four missiles left - use these to severely damage a second Wraith in the far corner, diagonally opposite of where you entered (you may have to venture out onto the bridge for a good vantage point.)

Destroy Destroy All Other Wraiths

Once you're out of missiles, hop back into your Ghost and follow the walkway to finish off the second Wraith - a few shots should do it in. Now hang back by the entrance into the room and blast the enemies within.

There's another closed gate here, so proceed up the staircase instead -- except there are more enemies there. Draw their attention with gunfire, then hang back to pummel them (a well place grenade can be effective too). Dart up and turn right and follow the path into the next room. Remain by the entrance, as a horde of Drones will soon fly in. Fire at them through the doorway. When clear, continue down the hall to a row of waiting Mongooses.

Before hopping on, scout out the ammo cache on the right to swap your weapons out for a Battle Rifle and Rocket Launcher. (We strongly suggest grabbing a rocket launcher now, as you won't be able to return to this room). Now hop aboard the driverless mongoose and tear down the ramp into the field.

Look for another anti-air Wraith on the field's far side - get behind it and let your rocket-launching comrade do the hard work. Shortly after it's destroyed, a few Phantoms will unleash several Ghosts - we suggest camping near the fallen Wraith and let your teammate do his thing. Once the arena is cleared out, a giant Scarab will roam on in.

Kill the Scarab Tank

As you probably guessed, the Scarab is a force to be reckoned with. Its head is equipped with a massive energy beam that will destroy anything it comes in contact with - avoid this at all costs by staying beneath or behind the Scarab. However, like the Death Star, it has a glaring weakness: the easily destroyable control room near the tank's rear. However, getting to that control room can be tricky.

Normally (and such is the case for future Scarab battles), you would want to disable one of the Scarab's legs with sustained weapon fire (preferably either rockets or Ghost fire). This will cause the Scarab to drop to the ground, allowing you to board a loading dock on the back, and work your way to the control room. However, there's a much easier method to besting this first Scarab.

Instead of fighting it directly, look for a lift at the base of the crane (along the wall). Interact with the switch to ride it up to the top of the crane. Walk to the far edge and sit tight - the Scarab will soon walk by and park itself right next to you, allowing you to simply jump on top of it, saving you tons of time and ammo.
(Thanks to Vitaman for the tip!)

Now be careful, as there are several enemies aboard. Use your rocket launcher to take down the Brutes and work your way to the back of the Scarab to find the control room. Melee the shield to destroy it, then melee the console twice more to take it down. Now you only have about 10 seconds to get the hell off the Scarab before it explodes, so move fast.

Neutralize the Anti-Air Cannon

With the Scarab down, head inside the building, via an entrance diagonally opposite of where you entered the field. You'll soon come across a makeshift command post; switch weapons with the soldier carrying the Rocket Launcher to prepare for a next coming up.

Now wind through the halls until you emerge in a room full of sandbags. Hang back and target the enemies beyond with your Battle Rifle. When clear, climb a staircase on the left up to a walkway and grab hold of the turret and spray any enemies that walk on through. Rip it off afterward and continue into the room ahead.

Use the gap between the two large boxes to shoot at the enemies and dodge to the side if you begin taking fire. Once clear, continue into another room.

Here you'll see a group of technicians get blown away by a pair of Hunter's energy blasts. Quickly jump onto the large box, just right of that area (use the smaller, nearby box as a stepping stone to reach it) to get out of the way. Now whip out your rocket launcher (you did grab it before, right?) and fire a rocket or two to ambush them as they run in.

Now drop down and shoot at the Brutes beyond. If the hammer wielding one runs toward you, launch a rocket if you have any spares, or backup and riddle him with battle rifle bullets.

Head outside and climb the ramp to battle a few more enemies. Now continue on to a vast hill, where you'll find the huge anti-air cannon at the top. Before climbing the hill, look for a sniper rifle on a platform to your left, in front of the mortar. Use it to snipe whatever Brutes you can see ahead.

Climb up the left side of the mountain, take down a Grunt manning a turret, then continue up to a rocky ledge on the left. From here, use whatever remaining sniper ammo you have to target the Brutes across the way. Strafe left and ride to evade their plasma blasts, or backup down the rocky slope if you begin taking fire.

Now here's the deal, you don't have to kill every enemy here, only the ones preventing you from targeting the anti-air turret's weak point. Speaking of which, look up to find the weak point on the base of the turret - it's an egg shaped obtrusion that pops out whenever it fires a blast. Fire shots at the glowing part, on its backside to destroy it (the nearby turret, mentioned in the above paragraph is ideal here). Once destroyed, the level will be complete. Nice!