That sounds like a good plan. Though we need to keep in mind that we still have another group of Space Marines up above. If the Orks keep going up, we could feasibly hit them from the rear. If we coordinate it with the upper floor group, we could catch them in a pincer manouver. Or we could keep going down the other stairway, and hope that the ones we left on the upper floor will be able to handle the Orks who are coming up the stairs :)