You can do this on Casual difficulty. You can also have bots. So to make it easy, do this.
When you spawn in, check the two new areas (Main Offices, Central Filing) if the Mystery Box is there. That will give you better guns and make this much easier. If not, I suggest restarting until it's there.
Now, you can open all doors you want, even the elevator, but don't activate it to go down to the War Room. The bots might open doors but they won't turn on the elevator themselves.
So that is my strategy. Go sit in the elevator. The bots will roam around and do their own thing, however when you enter the elevator they will all join you. So you got three bots with a lot
of health and ammo protecting you. There were rounds I didn't even move or shoot.
You gotta help the bots on the later rounds though, the dogs are strong in this game. Luckily your grenades are too if you there's a lot of them in front of the elevator, make them go boom.
This is really easy this way, and takes about 30 minutes to do. Good luck!
I recorded myself doing it so you can have a general idea of how it works, but since I sit still it's quite boring to watch: