You could take care of Kessler but his room is the evacuation place so unless you finish other objectives, you still have to go back to his place.
The factory (M6,23) can easily accessed by the main asphalt road going from the south (M6,19). The road (if you followed the guide) should be clear. But be careful, because from time to time the tank passes the road.
In the south, near the entrance, there are two Jagers and in the factory there are three/four enemies (as long as you are quiet). Inside, everything should go smoothly because the machines makes a lot of noise.
Again, you can sabotage the factory in two ways. Below, from the west entrance there is the machine that can be used to neutralize all elements in the factory (M6,20).
The second option is to destroy all blue mercury tanks (do this from the distance), there are six of them. If you go from the south you can destroy the tanks to makes things easier. Remember that when you destroy them, you won't have anything to mask your shots.
Finally, take the document from the safe (M6,21) on the floor. You can find it in a small office behind the turn. You must blow it up. If you do it, be aware that the soldiers from the square will enter the factory - the best way to avoid them is to use the small corridor on the ground level.
Finally, go to Kessler (M6,22). He is guarded by three soldiers. Two of them can be killed by blowing up the cistern and the third one must be quickly shot - when you get to building and speak with Kessler the mission is completed so you don't have to worry about any alarms.