Note: I'll keep things as spoiler free as possible but some things might be given away if you think too hard on it. I'll but some general tips for the last bit at the bottom just in case you want to wait until you reach the target to read that. I know other guides for the level go into more detail, but I'll keep it vague.
This is more of a clarification than a full guide. You will need Silent Assassin/Suit Only and all of the normal level challenges. Just make saves as you go and look at the challenges as the level goes on and reload saves to work on different things. You should only need to play the level twice and reload just the very end 2 more times because you need suit only and all outfits.
The only thing you really need to reload for is the very end. Which is explained with few spoilers at the bottom.
-With the recent update you will need to complete Silent Assassin/Suit only. This isn't difficult while managing save files. There isn't really any special path to take. The level is very linear.
Some other tips spoiler free tips:
-The code at the start is 1979.
-There are two shortcuts. The first you go to the next window and go back and the second you move forward and get a valve to open the container.
-Pick up the 2 big weapons, use the work bench with paint cans to make a silencer and use all 3 on enemies.
-Pick up a frag grenade. There are plenty in the level.
-Push a guard off the side. You can dump a knocked out guard if that's easier.
-Shoot a signal above the train. There are a lot of these.
-Knock out 3 guards with soda cans. There are several cans.
-There are 7 outfits. You should get most if you are sneaking around. They are pretty easy to spot. Keep plenty of saves if you miss one.
Possible spoilers for the last save:
Save before taking out the last target and you can reload after seeing the ending. There are 3 choices at the end: gun kill, inject, and, hold the thing from the case at the end for a minute instead of killing the target. After not using the item, there will be a prompt and then the a different ending you can watch.