Is it any better than Haste, Fireball, Lightning Bolt, ect.?
Its a good defensive spell, but its certainly not overpowered at third level. If it gets cast and renders a fight trivial, whats the problem with that? Good prep and/or a good choice in spell selection should be rewarded once in a while. How often does Haste dominate fights? Just about every time its cast.
Also, unless the monsters in question only deal one kind of energy damage it shouldnt totaly trivialize it. Have the baddie make a spellcraft check to ID the Mass Resistance being cast and then have him select a spell he feels the party probably didnt protect against.