IS BALLS OUT DIFFICULT!
Make sure to bring plenty of food, water and jet for this mission. If you have psycho-jet, that's even better. If you can, have unique foods like deathclaw steak for a small agility boost, radscorpion steak so you can withstand the attacks from flame throwers and Yao Guai Roast so your melee attacks will hit like a loaded freight train.
This place is guarded by some insanely aware guards, night and day makes very little difference to their ability to spot you from far away. Maybe use a sniper rifle to pick off one or two, but know that you'll have a swarm of tough as nails flamer wielding brutes known as "forged" on your ass soon, so have a plan if things go south. Maybe lay some mines prior to your siege.
If you can sneak close to the facility, there's a small shack on one corner of the back of the building. When you open the door to that shack, it'll set off a flamer trap which you can just wait out. If you don't mind playing dirty, you can sit inside the shack by the closed door and use VATS to kill anyone who opens it and charges inside. After killing one, shut the door and wait for the next one. After it seems like you've killed them all, scout the entire perimeter carefully. There will be some on the roof that aren't worth your ammo, but just make sure that there aren't any lurkers waiting to ambush you. There isn't much loot beyond some common resources in the small shack and whatever goodies you find on the forged.
In the entryway to Saugus Ironworks you'll find a few mattresses, definitely save your game before going inside. The forged that are inside near the entrance weren't as tough as the ones outside for my play-through. I recommend sneaking left until you can go up a metal walkway and get the drop on a lookout forged. There's two more forged on the floor of that part of the foundry, they should be easy pickings using either melee or a long range gun. You want to kill these first few forged without alerting the entire facility to your presence. Make your way to the rear of this area and find the stairway up, from up top you can scout out where the forged are in the next section of the foundry.
If you have a companion, they'll probably get themselves killed in this next area without careful supervision on your part. It's not a big deal, since enemies swarming on your companion can make them an easy target for a grenade. There's a gun turret in this area, that's usually what incapacitates your companion as they mindless attack the patrolling forged. After killing everyone and everything, make your way up and over to a door leading to the "blast furnace" area. If you are capable of picking an expert lock, I recommend taking the door to the roof instead of the door to the blast furnace.
On the roof, you can easily find cover to kill the three or four forged that will be patrolling around there. There's a mattress in one of the huts, save your game and be ready to deal with a world of shit.
After entering the blast furnace area, you'll have to talk down a kid named Jake who's being coerced into killing an innocent settler, you may have been asked by a settler to find Jake already. If you fail the first persuasion attempt, DO NOT attempt to persuade him again. If you fail twice in persuading him, he'll join the forged and team up with them to annihilate you. The big bad guy -- Slag -- will say his piece, and give a three-second countdown until he expects the kid to kill the peasant. Nobody will attack you while he's talking, even if you run up on them. Take a hit of jet, run up to Slag and start dropping mines at his feet. Everyone else will just hang out while you do this. When Slag counts down from three, they'll go off, so don't linger too long. When you've dropped enough mines to satisfy you, run past him and up higher on the catwalks to find a forged chilling out with a view of the whole area. Set off the action by attacking this forged, and the mines will erupt around Slag. Slag will probably only be weakened, but you'll likely take out the forged that's standing next to him. You can get killed out of nowhere by a wayward forged in this encounter, and slowing down time with jet is about the only way to stay a step ahead. If time is slowed, Slag is much easier to kill. After dispatching the forged, talk to Jake to further the quest to find him. Loot the Shishkebab off of Slag, grab the explosives Bobblehead from near where he was standing, grab the Picket Fences mag from the area below where he was standing, and search in the back area for a "dampening coil". The dampening coil is needed for the Here There Be Monsters quest.
Make a few trips back and forth from the Greentop Nursery so you can pick Saugus Ironworks clean of resources.