The Wonderwaffe, at the window upstairs behind the power switch and a buddy at the window and two on the stairs FU&^%$# rocks. I would even settle for an upgraded flame thrower at this same window, it turns them all into one shot kills as the reach the stairs for your buddy. So either of these guns are my choice...
They all rock, but you need some heavy fire power in late rounds, so i have to go with An upgraded flame thrower on a window and a Waffe when your buddies cant hold it down with out you.