Mission 3
Personally I found this a pain. I was having trouble with getting the soldiers to come closer or spawn in quick enough. So....
I had bullet time equipped and upgraded as much as i could. After defeating the combat drone you are tasked with pulling a lever. As you go to that room 3 cyberdogs will spawn. Ignore them and let them run out of the corridor.
Now run down the corridor and hit
to pull the lever ignoring that zombie for now. Get back to the corridor and ignore the cyberdogs. Now activate bullet time ad kill the 3 cyberdogs and zombie as quick as you can. Now head out the door asap and kill the cyberdog on the stairs to the right of where you come out of that door and another will come down the stairs to your left.
This may take a couple of tries as I think using the turret was harder. Hope this helps.