Cast Physical Wall, then dodge to the side. Check the side and south as well, see that nothing's moving there, please.
Wall: Do the doors open inwards, GK? If so, put the wall two lines behind the X, from E wall to W wall, in the middle of the doors so they can't be opened
If not - drat! -, put the wall one line behind the X so as to block off the re-enforcements from their side of the room, at least temporarely.
Question: How customizable is the wall? Could he, say, create it one foot shorter than the ceiling? Or close-off each door individually (basically straight across each door and along the backwall in between)?