Recruit Nino with a lord, then have Nino recruit Jaffar, make sure to have as many as possible of your units with torches and make sure they use them whenever they arn't attacking or healing other units, bring healers and defensive units mainly. Move quickly and open all the chests, then fold back and put your units in guarding positions, stopping enemies from entering Zephiel's room. Fire Emblem Wikifor more info!