I'm also not against people going free choice. The deal is, due to instant damage that Black Mage causes, theres virtually no risk in some encounters(Divine Might.) The best solution would be my resist idea. A real-quick temporary boost to magic resistance after you're hit with one of the 6 elemental DD spells. The only way you could possibly get around that fix would be to land all nukes at the same exact moment. Which due to lag and human error, is nigh impossible.
Could you still stack? yes. You'd have to time your nukes seperately. Would I have a problem with that? No. Because theres still the risk of losing. Just like MNK stacking in Ranperres. It's not 100% safe.
Black Mage stacking is just taking the easy(broken) way out, avoiding any challenge(risk) and any potential fun.
Is winning fun? Yeah.
Is winning and making an accomplishment more fun? Ofcourse. Theres a lot of fun in earning the right to yell "I showed you!" to Square.
Dropping nearly 30000 damage on 5 mobs all that the same time, isn't as fun as narrowly avoiding death to do it over time.