The Barriers are quick and relentless with their attacks. They use powerful spells like Flare and Holy, which they reflect off themselves to take down the player's party.
The quickest way to deal with the barriers is to have a Summoner cast Bahamut. If the player went out of their way to get the Mime job, they can speed up the process. Whenever the barriers attack, the party's White Mage should heal everyone. Once the barriers are gone, Necrophobe should not take too long to defeat.