Baba Yaga DLC - Collecting the Chemicals from the Copper Mill

Go through the gate southeast of the Copper Mill Yard Base Camp.

Climb the yellow ladder onto the boxcar. Jump across to the roof of the next building.

Then jump into the doorway of the building ahead on the left.

Inside you'll find a box containing the chemicals.

As Lara mixes the antidote, Nadia warns her of an approaching enemy patrol. Turn around and take out 2 men who climb onto the roof ahead.

Then either jump over to the roof where they were standing and deal with 3 more Trinity soldiers down in the yard to the right...

...or shoot them through the windows of the room where you found the chemicals.