From Halopedia, the Halo wiki
Part 01: Ladies Like Superior Firepower
Take the “sixty-six tons of straight up, H-E-spewing dee-vine intervention” that Johnson gave you. The Warthog is nowhere near strong enough to survive the attacks you are about to face. Destroy all of the Ghosts that come your way. Be sure that they are completely destroyed—if you leave one alive, it will fire on you and you will have little chance of surviving. Take out the Wraith as soon as you have a clear shot. Be careful not to fall into the hole as you drive across the bridge. Take out Banshees as they fly by. Destroy anything at the tunnel entrance below, then look toward the skyscrapers because six Banshees will fly from them.
When the Banshees are scrap, drive into the tunnel. You will encounter minor resistance; take it out. When you reach the first partition, get out of your Scorpion (which should be nearly a wreck) and take the Marine's Shotgun. Exchange it for whichever of your weapons is not the Battle Rifle.
The next section is full of infantry. Jump down and take cover behind the truck on the left side of the tunnel. Watch your motion tracker and put an 8 gauge into any enemy that comes to your side. (For most Elites, it will take 2.) Headshot the Jackal Snipers before they can kill you. Keep advancing through cover until you reach the next partition. Take a Jackal's Beam Rifle (in exchange for your shotgun), then enter the tunnel on the right.
Part 02: This Town Ain't Big Enough For Both of Us
Assassinate the Jackal Sniper ahead of you. Stacker will ask you to get on the turret, but you will probably get sniped before you can. So instead, strafe behind cover, sniping Jackals as they show up. They are coming out of an alleyway, and they will continue to do so until you occupy it (a rare case of infinite enemies). Advance to this area, but take care not to get sniped. When you occupy it (and kill any remaining snipers), kill the rest of the Covenant. If the Warthog has not been destroyed, an Elite will probably have taken it—and he will fire the cannon at you. In that case, snipe him before he can kill you. Two Ghosts will show up; take them out.
Alternatively, before proceeding to this area, you can backtrack and retrieve the Warthog the two marines left in the tunnels. The Snipers will have a much harder time landing a hit on you while you are driving it, and while you are completely unable to fight back while inside, the vehicle can provide a safe and quick method of bypassing the Jackal Snipers and heading to the next section.
In the next section, you will encounter the Wraith and more Ghosts. Grab a Rocket Launcher and put the Wraith out of commission, then destroy the Ghosts. If you run out of rockets, stick the Ghosts or snipe their drivers. Snipe the infantry near the waterfall, and be careful for the two Jackal Snipers.
Go behind the waterfall and give supporting fire to the Marines.
When the Ghosts are down, drive around the bend in the road. Kill the snipers on the half-bridge, then turn left. Use this path to flank the Wraiths; there will be a rocket launcher to speed things up. If you have a vehicle, get behind one of the Wraiths and start gunning it down. If you don't have one or would like to upgrade yours to a Wraith, try to flank it without being noticed, then get on the hatch (not the back) and melee until the Wraith is yours (don't plant any grenades). Let the other Wraith fall to your plasma mortar or whatever you have. Then deal with the minor threats.
Once the Wraiths are scrap (or one is and you are driving the other), go into the hallway and follow Perez to the balcony.
Part 03: Field Expedient
Once the Scarab passes, go into the building behind you and up the ramp. At the top, you will see the Scarab again.
Bombard the deck with all of the HEAT missiles around. Once those are used up, snipe anyone remaining on the deck, then grab a shotgun and jump on board. Enter the control room and stick the Elites. Use your shotgun to kill anyone who survives and end the level.
Okay, so here is the level walkthrough for the level Metropolis, on Normal difficulty.
Speed past the few Covenant guys at the beginning of the tunnel, they will not chase you. Eventually a Warthog will come down from a tunnel, and speed up towards a barricade. Let your gun and Heavy weapons Marine take out the infantry on top of the little wall. Then, get out, swap your SMG for a Shotgun. Keep your Battle Rifle, you'll need it.
Drive your 'Hog over the little hill and into the tunnel again. Get out and get in the turret. Elites are no match for this gun, and Grunts will easily be disposed. Keep driving it through, getting out, and shooting. Watch for the invisible Elite, but if you see his outline, you can stick him, and he's toast.
Use your Battle Rifle to zoom in at the Jackal Snipers, run close and they panic, giving you the chance to melee them at the back. The Elites at the far end of the tunnel will be no match for your Shotgun. When you kill the last guy, go back and try to find a Rocket Launcher, which you can trade for your Battle Rifle or you can give one of your Marines a Beam Rifle. Keep the Shotgun and head into the pipes.
Swap to the Beam Rifle, and melee the conveniently placed Jackal Sniper in the back of the head. When he falls immediately zoom in and headshot the Grunt manning the turret. Continue to snipe out the rest of the visible enemies, then jump down and see if you can find the Gauss Warthog.
So, jump into the Warthog's driver seat, get Sergeant Stacker to man the turret, and drive off. Make sure the rest of the enemies are dead, then continue through the holes in the wall into a courtyard. Watch out for the Wraith positioned here. Drive straight past, and around the fountain. The enemies will not chase you. Then, go back and pick up the Rocket Launcher ammo stored near the entrance to the second large courtyard. Then, drive back around the fountain and get ready for some fun.
Turn up to the right to find a pair of Wraiths besieging A Company, who have taken shelter in a command post next to Tterrab Industries. Drive the Warthog to avoid enemy fire and blast the pair of Wraiths, then aim up as a Phantom attacks. Fire a couple of shots at the bottom of the Phantom to destroy the plasma cannons and the pair of Ghosts. The Phantom will retreat. You can also board the Wraith, but you'll leave your Warthog to get blown up.
Now go up the back alley to the left to find a Rocket Launcher plus some useful ammo for your Shotgun. Take it, then go back to the bunker to make contact with Corporal Perez. Go up to the command post to meet Sergeant Banks, and watch a Scorpion Tank get destroyed by the Scarab's mighty Scarab Gun. The Scorpion can actually be saved, and the Marine driver will act as though he's holding a gun, but he isn't.
Okay, inventory check: Battle Rifle and fully loaded Rocket Launcher. Got 'em? Good. Don't got 'em? Get 'em. Another choice is the Shotgun and Rocket Launcher, play co-op and you can share.
Go up the stairs to the left, and come to a door where a Marine will be waiting to escort you. Go up the stairs, onto a large water channel. This is ammo heaven. There are tons of rocket ammo boxes, so stack up on those. If you lost your Shotgun, get a new one. DO NOT JUMP ON THE SCARAB. It will stop at the end of the canal, so no worries about it escaping. Enemies will keep coming out, aim your Rocket Launcher and aim a bit forward to make sure you get your mark, they'll drop like flies. Go to the last platform. There is a turret there. Get on it and start shooting. The Scarab will make its way around the corner soon, so get ready.
If you didn't use your Rocket Launcher to mop the enemy, use your turret to take out all infantry on the top deck. The Elites with Plasma Rifles will be your biggest threat, but will be no match for your turret. Finally, hop on and take out any resistance left on the deck. Then, go below deck and fire some rockets at the Elites and Grunts. One will come upstairs with an Energy Sword. Either stick him or use your Shotgun. Then, go downstairs and take out the remaining Elites at a safe distance with rockets or sticks. The cutscene will then appear, indicating you have beaten the current mission and objective.
Speed Run Walkthrough
Hop in the Warthog, let the Marines jump in, and drive, ignoring anything in your way. Except for the giant hole in the bridge. And the barriers. Anyways, reach the end of the bridge, drive through the tunnel, and reach the massive barrier. Drive throug the narrow hole to the left, down the ramp, around the wrecked vehicles, up the ramp on the right, and through the pipe leading to the jackal snipers. Once you reach the Gauss Warthog, toss Sergeant Stacker out of the driver's seat, hop in, wait for him to get back in on the turret, and drive to the exit. Drive around and have Stacker blast any of the Ghosts and Wraiths in the way. Don't bother waiting around for them to be annihilated, just drive through. Once you've exited, head right, then left, then blow up the Wraiths. Jump out of the Warthog, head up with Corporal Perez and Sergeant Stacker, and meet up with Sergeant Banks. Run up to the massive armory and grab a Shotgun or Rocket Launcher, and a Sniper Rifle. Once you reach the gangway area, head out to the overpass closest to you. Once the Scarab is underneath you, jump down onto the roof of it. Sneak around to the sides, using the roof as your central point, and kill off the waves as they come up. Alternatively, you can sneak past the enemy into the cockpit of the Scarab. Once below, use the Sniper Rifle and either the Shotgun or Rocket Launcher to decimate the Covenant. Leave the Elite Majors (the pilots) until the very end, as they will not attack you unless provoked.