ONI Alpha Site/Walkthrough
From Halopedia, the Halo wiki
This is the second mission that you play as Dutch, with Mickey as your ally. Your mission is to defend the ONI Alpha Site facility and deny access to the Covenant. This mission is one big firefight. Your starting weapons are the Spartan Laser and the SMG. There is no subtlety or sneakiness to this level. You cannot avoid the enemy in any part of the game, so knowing your weapons, their capabilities and how to best use your NPCs is really important. There are no "good" Skulls for this mission, but Black Eye, Tilt and Mythic will make this level feel nearly impossible to beat and incredible when you do.
Boom Goes the Dynamite!!! (Or, Why I love my job!)
As you start this mission, you're running away from the Covenant. Get used to it, most of this mission is running away. Arm the charges, then head for the left hand tower and set them off, blowing the bridge and killing everything. You can run around shooting at enemies and eventually Mickey will detonate the charges, but on Legendary, you can't afford to waste the ammo. Head over to one of the Gauss turrets, and take out the Wraith on the other end of the bridge, you'll end up shooting two Banshees too. You could skip this, but occasionally some Plasma Mortars will make it over the wall. Don't risk it, take a few seconds and clear your enemies. Once done, head under the wall with Mickey.
You Wanna Piece of This!! (Or, Come get Some!)
Under the wall, Melee the Jackals before heading out. Bring up the Spartan Laser and quickly dispatch the two Hunters. If you're lucky a few others will stray into the beam. Start running. Head towards the building shooting and tossing grenades. It will give you a little relief when you're running to the shelters. Once on the first tier, swap your SMG for an MA5C. There's plenty around and on legendary difficulty there will be plenty of dead Marines to get ammo from. The MA5C is better at close range than the SMG and has a better melee. On lower difficulties, stick to the SMG, the Marines are heartier and won't die so you can't get more ammo, plus the SMG deals death pretty well on lower difficulties. As you withdraw, use lots of grenades if you have them, as there are plenty around. Once you're at the top, stay away from the turrets as getting behind a stationary emplacement at this point is instant death, run down the stairs, grab the Rocket Launcher behind the wall and give it to Mickey. He'll do a lot more damage with that than the SMG. Run back and grab your Spartan Laser. There seems to be three ways to trigger the recall into the building:
Bring the Rain!!! (Or, Lead Showers and Bloody Flowers)
Once your inside head over to the center and drop your second turret towards the back stairs, it will be safe there and you can come back for it later. Take your place at the second turret. Follow the directions of Mickey and the NMPD officer, but don't be beholden to them keep sweeping left and right. You can kill the Kamikaze Grunts and Jackals, but watch out for the Brutes with Brute Shots. Hop off the turret, and wade into the fray, using grenades and your Assault rifle. The Brutes seem to focus on the center, so if you stay to the top you can sneak around and assassinate them from behind. This isn't foolproof though so stay alert. Between Mickey and yourself you should be able to end the frontal assault quickly. Next, grab one of the turrets and watch for the Phantom to drop its troops behind you. Use the turret to clear the Brutes and then finish the rest off with your MA5C, taking out the Jackals and Grunts. Mickey's rockets will do plenty of damage to help you out. Once you're done, trade a Brute Shot for your MA5C, keep your Spartan Laser and the other turret. Grab any grenades you can find and head on to the elevator.
Enjoy a Firefight on Our Beautiful Rooftop Patio..
Before you start the elevator, drop your turret and swap your Brute Shot for the SMG on the floor. Pick up the turret (it can disappear), and activate the elevator. Drop the turret near the door and stay clear of Mickey and the cop as they shoot their rockets and grenades at the Drones, clean up what they miss. As you reach the top, go back for the Brute shot and pick up the turret. The heavy weapons will make short work of the jump Brutes here and you can then focus on the Chieftain. Between your laser and Mickey's rockets, he won't last very long. Hop on the Pelican and watch the fireworks...
Notes for Lower Difficulties