What I've been doing is clearing out everyone as they approach Nah's entrance, and have Gaius and Anna paired up. I clear out the hall, have Chrom get Nah, then have Libra boost Luck and send Chrom/Sumia, Lon'qu/Vaike, and Gaius/Anna through the hallway, get anyone to the one spot that's safe, if possible, and hope for the best.
Move down to the chests, deal with the unit there, get the chests open, and then slowly bring the rest of my units through the hallway and have Anna/Gaius (whoever's leading) open the door.
What happens to me is that the room next to Nah opens then, and the units I didn't send through yet get stormed and slaughtered, yet I fear sending them through the Mire hallway, and I don't want to leave too many units behind.