Destroy a crystal, if they are all already awakened in the final chapter

The earliest class able to attack multiple enemies without using MP is the Valkyrie, obtained in Chapter 2. Their initial skill is Crescent Moon, which deals conventional damage to all enemies and costs 1 BP.

There is a trick to Orthos - if you use an opposing element on a head (fire on ice, ice on fire), that head will 'shut down', making it unable to use any elemental attacks and switching to physical. It also eliminates the possibility of using Blazzard, because that requires both heads to be 'active'. This state lasts about 3 turns, after which (or during the third, I'm not sure), it will re-activate, possibly casting its element's AOE. Because of this, never disable both heads at the same time, always go for one first and the other a turn or two later. This way Orthos will be completely unable to use Blazzard, as long as you keep disabling the heads again and again.

My suggestion would be to have your healer (or whoever will go first) use an offensive item (either fire or ice, you should have at least one by now) and then just let a black mage to roll with it. However, if you feel like you still aren't making any progress, try leveling up, both party members and their jobs. If you haven't done the sidequests yet, do so, they can serve as a sort of a benchmark.