The first thing you will notice during this mission, if you normally have your UI set to mini-map, is that it isn't working. This is due to strong electro-magnetic interference and high levels of radiation in the area. You will simply have to do without. Another thing to note is that scurry robots are very frequent during this mission. They will pop up in almost every other area.
You begin the mission in a snowy field. To the right is a small doorway to a closed in area. The only thing here is two humanoid robots. If you are wanting to complete the optional objective, kill them both. If not, skip past them and head to the left. Soon you will see a small camp, some graves, and an enticing ceramic pot. Inside this pot is only a Cherry Nuka-Cola, and x50 .50 cal rounds. If that is enough to lure you over there, you will be ambushed by around 5 scurry bots. If it isn't worth it to you, simply skip past it and continue until you see a dried up river bed. Next to it, at the corner of a map, is a small bricked off area, which will have a humanoid robot waiting. Again, if you are doing the optional objective, pop in and kill him, if not, follow the river bed south. Soon you will be fired upon from a bunker by four more humanoid robots, armed with two sniper rifles and two P94 plasma rifles. You can use nearby rocks to use for prone cover in order to advance closer to them if needed. After this head through a door in the fence at the south. Here will be more Scurry robots and more humanoid robots in trenches, two of which are using plasma grenades. Continue through this area until you get to a trench with a control panel (and two humanoid robots). This control panel deactivates the force-field doorways you've been passing by. Now you can get to the main door to the factory's entrance ramp. You can get to this from either the doorway right next to you, or through the doorway further to the north that you passed earlier. It is suggested that you take the northern gate, as the gate next to you leads to a Behemoth. This is best avoided unless you are doing the optional objective (if the case, see more on the Behemoth in the Optional Objective section). Once at the main gate there will be another console. This one will require you to use your science skill, which will then open the gate. As soon as it opens you will see the remains of Paladin Lancelot and most of his squad. Take care of the auto-cannon in the doorway and head down the ramp to enter the factory.
When you approach the bottom of the ramp, move the one of the corners of the doorway. If you enter immediately, you will be ambushed by 6+ security robots, which when all firing in unison, add up to quite a bit of damage. Standing in the corners lets you fight them in pairs of two. After that is taken care of, you can now destroy the first power node. Unlike the previous mission, they do not auto-repair. They also have quite a bit more health (around 15 times as much), so it may seem like shooting it doesn't do anything, but it just takes a while. Once it is destroyed, head through a doorway on he south side where you will find another console. This will play a message from the Vault 0 logs and mention the Hellion project, which will be found in a later room. The door behind this console leads to a room where you can find Initiate Percy, who is near death. After a few last words, he succumbs to his wounds and expires. Now head into the hallway, where you will meet a few more robots, including a loadlifter robot. Across the hall is a room with another console.
This console will open the door at the end of the hallway, allowing you to progress. Go up the staircase and you will now be on a walkway with a view over the next few rooms. Here you can snipe the robots from a distance, including three Loadlifter robots. When finished with that, be cautious as you continue down the walkway as there will be 6 Security robots and a humanoid robot with a Vindicator minigun waiting for you near an alarm console. They are fairly grouped up, so burst fire or grenades will take care of them quickly. Continue up the stairs and wait a moment. At the end of this walkway is a laser turret that has been modified to fire rockets. You can either kill it or wait for it to go below ground and dash for the ladders to the ground floor. Cross the room and you can either proceed through the doorways or you can climb a ladder up onto another walkway. Climbing onto the walkway gives you the tactical advantage over the Robots in the next areas, as lying prone on the high ground will make it much harder for them. If you climbed up the ladder to fire from above, climb back down and head through the doorways. Now you will be in the room with the Hellion. It isn't actually related to any objective so you can ignore it. There are three paths to take from this room. A door at the left, an opening at the top, and a door at the right. Since you have to go through all three, it doesn't really matter which order you choose. To the left will be a room with more laser-turrets, except these will be more true to their name and actually fire lasers.
There is another door at the top of this room, which leads to another power node. When it is finished off, return to the Hellion. Now for the door on the opposite side of the room. If you climbed onto the walkway earlier, this room will be empty, but if not, it will contain several humanoid robots. At the end of the room is a door to the last reactor. Destroy it and again return to the Hellion. Now for the opening at the top of the room. This will be to a catwalk that leads to the reactor. It is guarded by a variety of robots, but isn't too heavily guarded. The reactor consists of two turbines and a single piece in the middle. Destroy all of it and head out to extraction. Though it is a nuclear reactor you just destroyed, it doesn't have any catastrophic consequences to the structure. So don't worry about having to race against the clock to get out of the factory.
The secondary objective, to destroy all the robots in the area, is fairly straightforward. Just kill any robots you see and make sure you check every room. This will mean that you will have to face the Behemoth avoided earlier in the walkthrough. If you want to face it, head to the entrance ramp of the factory and enter its yard area. As soon as you walk in, the doorway will throw up a force-field to prevent you from moving back through. However, it doesn't force-field the doorway to the south, so you can still retreat. Along with the Behemoth, there will also be several Scurry robots in the area making things more difficult. The Behemoth has quite a bit of health, more than any other opponent you've faced so far, and will be a challenge. It is best to fight him in turn-based mode as it will be easier to manage your group and use stimpaks efficiently and quickly. If you are willing to use an exploit to fight him, you can simply move through the small doorway previously mentioned. You can easily out-range the Behemoth, so if you move far enough away you can simply snipe it while it stands stupidly in place, or runs away. If it moves away, just shoot at it some more until it charges again and repeat the process.