ONI Alpha Site/Walkthrough

Level starts
You enter Dutch's vision, and Mickey will ask you to arm the charges. Simply get to them and press RB to arm them. Get out of the bridge and climb on the tower with a sniper rifle. Press the controls and blow the bridge up. There should be no survivors.

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. It drops down a Hunter pair and Grunts along with Brutes. Spartan Laser can instantly kill a Hunter with a direct hit, or if you took the sniper rifle before then snipe it in the back or head. Staying on higher grounds would help in Legendary. 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. Don't use the turrets up there because Wraiths will fire at the walls, killing you instantly in Legendary. Now go down to the gates, and you would 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 the must fall back due to the overwhelming forces. A turret is in the middle. Enter the turret and later, the Covenant will breach in. The first wave would be lots of Suicidal Grunts. Having the turret can kill them easily. The second wave would be still 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 you you have nice aiming or you're not playing in Legendary. Behind the lines, a Phantom flies by and you know troubles' coming. Detach the turret (if it's still intact) and start the killing. There will be Brutes and Grunts. Waste them and Mickey will ask you to get to his position.

Extraction point
Dutch and Mickey met a surviving NMPD in the a lift, and together they go up to the rooftop. Drones will appear and start shooting. There is actually a cover, that is the door. Staying there in Legendary would make no Drones hit you and saving ammo. Up there are lots of Jump pack Brutes and a Gravity Hammer wielding-Chieftain. The NMPD usually won't make it here. In Legendary it takes about four shot from Spartan Laser to kill it, but less if you gave Mickey a Rocket launcher. Kill the Brutes out there and a NMPD Pelican will arrive and you should get on it.

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.