Drop off the building and head for the fallen train tracks. Take those on up to reach the next area: End of the Line. Head through the twisting canyon that leads to the open plateau at the end of this area. In each of the more open areas, and many of the narrow ones, you will encounter a good number of Loaders. Just make your way through all of them to the South.
You will eventually find a drop in the path. This is the "point of no return" before the fight with Wilhelm. There is a One Way Fast Travel Station just before where the fight is if it proves too hard and you need a few more levels.