There are multiple ways to do this, but I believe this to be the most efficient and easiest. The first and most important thing you want to do is buy the door for 500 going into the elevator hallway and make sure the mystery box is in the main offices. If it is not, simply reload the game until it is. If you do not pull the Winter's Howl before the box gets relocated I would simply restart. You don’t have to, but I think it beats going into the later rounds and hunting the new box down.
Make sure you open everything on the floor you spawn on. It will allow you more room to move around and train zombies.
Once you get the Winter's Howl group and train zombies up so you get the most out of your shots. I knifed or shot the zombies because when I ran through them after freezing them I could not get my trophy to pop. So be safe and freeze them and switch to your secondary and shoot them to break them.
If you are having trouble staying upstairs like I did you can go somewhere else on the map.
This can be done on casual I wouldn’t use bots because you need as many zombie opportunities as possible.