Black Mage - the solution for everything?

[Tadacho]

Easy solution to this BLM stacking would be to make it so that other mage's spells also count to your own resists.

so if there's 7BLM all casting at once, the enemy will become nigh invulnerable after two rounds of spells.

Or, you know, this really doesn't need to happen. As part of a group that consistently uses parsing I know for sure that sometimes I can come out on top, sometimes it can be the arc, the drg, the mrd, the whatever. Gear and skill come to mind. If this is the case then mobs should also generate resistance to physical attacks when everyone is stacking archer?