The only things I felt like barrier was necessary for:

Rager Berserker Warriors - No question here, roll the deathblow dice, lose and eat a 600hp shot.

Ridiculously geared fire giant sword specs (AKA 1k hp flurries), kinda self explanatory, takes a lot of gear to get to this and people really should loot these guys more.

Hero offense shifters, particularly tigers - my last hero level AP was before 'excision' so a tiger was pretty much a death sentence if they had an air form to get me.

Conjurers - kinda self explanatory, they pump out stupid amounts of damage.