ONI Alpha Site/Walkthrough

Overview
This is a walkthrough for the Level ONI Alpha Site It is written for play on Legendary difficulty and has notes for lower difficulties.

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 a few Banshees too. You could skip this, but occasionally some Wraith Plasma 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 a 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, and you should 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 and Swap with Mickey. He'll do a lot more damage with that than the SMG. Run back and Grab your Spartan Laser. There seem to be three ways to trigger the recall into the building. 1) Run around killing indiscriminately until a certain amount of time has passed, 2) Kill the Chieftain with the Gravity Hammer (this one is really hard so don't bother unless you're really sure you can get at him) or 3) kill the Wraiths.  If you're playing on Legendary, you should be able to kill a Wraith with One shot from a Spartan Laser.  Do the one on the left first, then run to the right and kill the second one.  This should trigger the recall.  Before you run in, On the ground on the right side in back there's another full Laser, grab it.  And if you think you can get away with it, grab a turret too.  Head inside.

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 get behind and assassin melee from behind. This isn't foolproof though so stay on Guard. Between Mickey and Yourself you should be able to end the frontal assault quickly. Next grab one of your turrets and watch for the phantom to drop it's force behind you. Use the turret to clear the Brutes and then finish it with your MA5C, taking out the Jackals and Grunts. Mickey's rockets will do plenty of damage to help you out. Once your done, load up with a Brute Shot for your MA5C, your full Spartan Laser and the other turret. Grab any grenades you can find and head into 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 NMPD officer's MA5C. Pick up the turret (otherwise it has been known to 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, switch 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.........

Legendary Notes

 * Machine Gun Turrets outside the building plus user playing on Legendary equals dead user. Stationary emplacements are quick death, by grenade, Wraith, Brute Shot Beam Rifle, or some combination thereof. Removing one from an emplacement and running around with it is slow and there's too much climb, though if you stand still, it mows down Brutes faster than a Sniper Rifle, but standing still on this level can easily be equated with death.
 * Machine Gun Turrets inside the building plus user playing on Legendary equals victory. Using the turret inside the ONI building and knowing when to get off (preferably before grenades hail down upon you) will clear out all of the minor annoyances and some of the big Gorilla looking ones. Disengaging it and running around will quickly finish off Brutes with short controlled bursts. An extra turret if you can get one from outside without dying is a great assist for the roof.
 * Don't take out the banshees at the bridge part with your Spartan Laser. Those banshees hardly ever harm you, and the laser shots you use on them would be a waste, as you need them for the next section
 * On the way up the lift, hide behind the door/glass section, if you don't want to get shot by the drones.
 * When the enemies start flooding into the ONI building, make sure you keep strafing from left to right and back again. If you keep your attention fixed on one side without paying attention to the other, you will definitely die.
 * Since Mickey is indestructible, make sure he has the heaviest weapon on the level, (the Rocket Launcher) with unlimited ammo he's an invaluable assist to staying alive

Not Legendary Notes
"Face firm resistance from competent, determined enemies, but burn through enough ammo and you will eventually triumph."
 * Staying at one of the Machine Gun Turret emplacements outside is death, but with four turrets available, you can easily mow down enemies on lower difficulties until the recall is triggered. You may even be able to snag the chieftain and his Gravity Hammer.
 * Taking the Sniper Rifle from the tower isn't recommended, but on lower difficulties it's easier to learn how to shoot on the move without the scope to assist you.
 * On lower difficulties if you don't assault the Chieftain on the roof, eventually the pelican will show up and burn into his shields and armor leaving him easy pickings for you to finish off.
 * The description of Normal is the perfect add on note for this level. Burn enough ammo and you will win.

- Description from Halo 3 and Halo 3: ODST