I think there are too many problems with mines of moria for it ever really work with classes. The falling bridge is completely bypass-able by all zombies with the exception of the deformed, so even if all the humans did have parachutes, the zombies could just jump the gap (because you KNOW there won't be enough humans defending, it would be an exaggerated version of spacestation because everyone would think they were doing good by both standing by the door so they can rush through when it opens and shooting (and missing) at the zombies).
"An Air Force A-10 Thunderbolt II flew a show of force and expended flares for emphasis over Salerno after enemy forces attacked a coalition forward base. The arrival of the aircraft prompted an immediate retreat by enemy personnel."