I've done this on 'Training Ground - Hostage Rescue' with only one friend with me.
All you need to do, is go from floor to floor, so you can find the hostage. It is easier if you start at the top & your friend in the basement. Check all the rooms as fast as you can, then move to the next floor. You don't need to kill anyone, I even ignored some of the targets & killed the ones that were in my way. Just break one plank of a door to look inside, if the hostage is not there, move on. When you got the hostage, ignore everything and just run. But keep in mind that at the end near the drop zone, there is an enemy waiting for you (most of the time he's alone, but sometimes there are two of them). If you restart the lvl, hostage location will change (sometime it doesn't change, but don't count on it)
I will leave a link to a video, so you can see how I did it.