ONI Alpha Site/Walkthrough

Level Start
Dutch and Mickey got attacked with heavy fire from the Covenant. The only way they can save themselves is to blow up the bridge and run…

You enter Dutch's vision, and Mickey will ask you to arm the charges. Each charge is found on a pillar. Simply get to them and press RB to arm them. Get off of the bridge and climb on the tower with a sniper rifle. Press the controls (press RB) and blow the bridge up. There should be no survivors. Although, sometimes 2 wraiths will survive on the piece of land at the beginning of the bridge, so watch out for those. You can make sure they don’t survive by shooting them with one Spartan Laser shot each; but make sure you have 2 laser shots for the Hunters in the next area.

Behind the Walls
Dutch and Mickey fell back to the line, only knowing that taking out the bridge wasn't enough because the Covenant sent Phantoms behind them, attempting to breach their base…

The Phantoms drop down a Hunter pair, Grunts and Brutes. A Spartan Laser can instantly kill a Hunter with a direct hit in the gaps between their armor, or if you took the sniper rifle before, snipe it in the gaps of armor in its back or head. Staying on higher grounds would help in Legendary, so make sure that once you take out the Hunters to quickly get up the hill. Get across the fields and regroup with the NMPDs. Then hold the position until a NMPD screams fall back. Take the turret if you want; but be quick. Using the turrets up there without detaching them could mean you will die because the Wraiths will fire at the walls, killing you instantly on Legendary, so it would be better if you quickly detach a turret and use it to fight off the brutes and grunts on the hill. Now go down to the gates, and you will find a rocket launcher. Pick it up and use it, and give it to Mickey before finishing off all the rockets. Since ODSTs cannot die, giving them powerful weapons would make them really strong and help you on lots of killing. Grab a turret or something and start the massacre!

ONI Base Interior
Dutch and Mickey tried to hold back the Covenant forces, but they fall back due to the overwhelming forces…

A turret is in the middle. Grab the turret without detaching it. Make sure you are ready because once the Covenant start coming in, they won’t stop. It will be like a shooting gallery, having you switch from the left doors and the right doors. The first wave would be lots of Suicidal Grunts. Having the turret can kill them easily. The second wave would still be lots of Grunts and a few shielded Jackals. In Legendary, you may want to stop, as the third wave with Brutes would be coming. Since they wield Brute Shots, using the turret would be a problem. But it's okay if you have nice aiming or you're not playing on Legendary. Behind the lines, a Phantom flies by and drops off a couple Brutes with brute shots and brute plasma rifles. Detach the turret (if it's still intact) and start the killing. As long as the brutes get killed, you’re fine. Waste them and Mickey will ask you to get to his position, which is at the elevator in the previous room.

Up the Lift
Dutch and Mickey met a surviving NMPD officer in the lift, and together they go up to the rooftop…

Drones will appear and start shooting at you. Take cover behind the door of the lift, and pop up to shoot the drones whenever you feel like it. But it would be best to save your ammo for the following fight. This would be a smart thing to do on Legendary because after you come off the lift, it will leave you with almost no cover whatsoever on the rooftop. Once you get to the top and the doors open, get ready.

Extraction Point
Dutch and Mickey fought the Drones all the way up to the roof. But they won’t get out of this easily…

Now you made it. Get off the lift and find yourself outside. Once up, there are lots of Jump Pack Brutes and a Gravity Hammer wielding War-Chieftain. The NMPD officer usually won't make it here. On Legendary, it takes about four shots from a Spartan Laser to kill the Chieftain, but less if you gave Mickey a Rocket launcher. Kill the Brutes out there and a NMPD Pelican will arrive. Getting on it will end the level and show the Post-Game Carnage.

Trivia

 * Blowing up the Banshees and Wraiths on the bridge would cause Mickey to activate the charges and shows dissatisfaction.
 * On the bridge, you get two kills if you shoot down a Banshee with the Spartan Laser and three kills if you shoot down the Wraiths. There are two Banshees and three Wraiths that would make getting the achievement easier.
 * There is a hidden Rocket launcher near the gates of the ONI Base.