BLM in feast is great when the opposing team ignores you. Otherwise, you spend most of your time running away or having your spell interrupted by line of sight or the target has run out of range. You could keep alternating between fire and ice to do some small damage while moving since its considered an instant cast.

I switched over to SMN when I got into a higher tier as it feels like a big red target when you go as a BLM. We have the least amount of HP and we are capable of bursting if we are left alone, so naturally you will be beaten to death.