From Halopedia, the Halo wiki
For this flashback mission you play as Romeo accompanied by Edward Buck. You begin with the Sniper rifle and the Pistol. Your mission is to reach the crashed Pelican and assist your squad in repelling the Covenant. You can turn on nearly any skull you want, and get away with it on this level, but Black Eye is a risky one because you are so rarely in melee range of an enemy.
Drive-by in front of a Police Station? They must be crazy.
As soon as you start reload your sniper rifle. Use your sniper rifle and pistol throughout the first stage of the mission. Follow Buck, who will lead you to your first encounter with the Covenant. Edge out the door looking to the right. Take out the Brute leading the patrol, then use your pistol to kill the Grunts. You will find a Shade operated by a grunt on the left. Use your pistol or sniper here, one shot will clean it out. A Phantom will deliver additional enemy reinforcements. Take them all out.
Before you head through the door take cover behind the Shade and wait for the Phantom to leave. Keep an eye out for Grunts emerging from the door, but Buck will keep them at bay and even kill a good number of them. Once the Phantom is gone use your pistol to clear out as many enemies on the high ground as possible. You can take out the Plasma Turret operator and watch as Grunt after Grunt runs to take that position. Take out any Jackals you can see then go through the door. Move quickly out the other side and up the stairs where the turret is located and use the crates for cover. You will find additional sniper rifle ammunition here. Buck will clean out anything left on the lower level, leaving you free to smoke the Sniper Jackals with Carbines on the high ground with your Sniper Rifle. After dispatching the Jackals, walk towards the ledge and immediately retreat as Brute and Grunt reinforcements arrive out of the building. Use both your sniper rifle and pistol to dispatch all Covenant, but keep your distance.
Reach out and Touch Someone with 14.5mm of APFSDS Love.
Follow Buck into the next room, with the corpses of NMPD officers lying all around the room, load up on sniper rifle ammunition and try to collect as many grenades as possible. Edge out the far door looking up at the building on the left, you should see Sniper Jackals on the balconies. Take out the snipers here, there are three then turn right and edge out further, two more Sniper Jackals are on the bridge. A Phantom will deliver reinforcements in the form of Jump Pack Brutes. Sniping them will normally require two shots to kill, so don't bother aiming for the head unless you're sure you can get them in one shot. Also, make sure the Shade operator has been taken out. A Brute will always man the Shade after the previous gunner has been slain, so if you can get a good angle with cover, sit back and take out Brute after Brute that mans the Shade. After this battle, return to the previous area to fill up on sniper rifle ammunition.
Some pistol ammo and a Silenced SMG can be found along with a health pack before the next battle. Do what you did in the last section and snipe some of the Covenant from the Shade's position. Once you head through the door, kill the Grunts waiting for you, then snipe the Plasma Turret operator on the left stairs. A Sniper Jackal may sometimes take up position above the Covenant forces, make sure to take him out as soon as possible, as he has a beam rifle and can kill you in one hit. To kill the initial Brutes in this area, you can use a mix of grenades and sniper rifle fire, but don't use your pistol for removing the power armor; you might as well throw rocks at them if you use it. After the initial Covenant force has been wiped out, two Brutes with Maulers and a War Chieftain carrying a Plasma Turret will come out of the door ahead of you. Again, you can take them out from a safe distance with your sniper rifle. If you're short on sniper rifle ammunition, it remove their power armor with one sniper shot, and then kill them with a pistol shot to their heads. The Chieftain will be a tough nut to crack, so shoot his helmet two or three times to remove it, then finish him off with another head shot. Proceed through the door the Brute Chieftain came from and down the stairs. You'll run into two Grunts before the landing pad, but their backs are toward you, so simply melee them. On the landing pad you'll see a Grunt with a Fuel Rod Gun and two Sniper Jackals. get the Grunt first, then assist Buck in taking out the Jackals.
How Not to Land a Pelican 101. (Or, I Don't Think AAA can Tow This One.)
Jump down onto the "bridge" and run.
Halfway along the bridge some passing Banshees will boost to try to shake you off, but just keep to the center and you'll be fine. When you arrive move to the left and clear out any remaining enemies, then run up the ramp to rendezvous with Mickey and Dutch and grab a Rocket Launcher and a Spartan Laser. Many Banshees will attack your position, so take up position at one of the mounted Missile Pods The one to the left of the pelican when facing out actually has a better field of fire, but it just becomes a matter of preference so just start shooting. You have unlimited ammo from the emplacements, so don't be stingy. Shoot fast and hard, aiming at the Banshees and Phantoms as they enter your field of fire. If you need to hop off to recharge and heal up. There are a couple of Spartan Lasers and a lot of Missile Pods around the Pelican. Rocket Launchers and ammo can be found on ammo cases on both the left and right side of the Pelican. Also, you can take Mickey's Rocket Launcher, but like Dutch's Laser, it's better off in the hands of someone who won't run out of ammo. Keep fighting, a few Phantoms will drop off ground troops including one group of Jump Pack Brutes, but you and your team will make short work of them with your heavy weapons. Eventually Buck will warn you about the Chieftain and...
Notes for Lower Difficulties