Elemental Resistance Or MDT-/MDB?

With elemental resistance builds in this game, it's a make or break situation.

To make it work on anything at least ten levels above your level, you're pretty much going to need +300 for it to work. That 300 can be obtained by any means necessary (gear/songs/spells/etc), as long as you hit that number or higher.

If you don't hit the sufficient number accordingly, then it treats it as 100% unresisted, which then does full damage obviously. In other words, it practically acts as if you just ate a magic-based attack with no elemental resistance gear on whatsoever.

Because of this and the vast amount of support an elemental resist build requires, the way to go is to equip magic damage taken -% gear until capped (25% in gear is enough if you're with 3/5 merited Shell V, 24 if with 5/5), which then from there you would throw magic defense bonus in the other remaining slots accordingly.

Without Aegis, all jobs are pretty much gearing entirely toward magic damage taken, since most (if not all, bar Samurai and perhaps Red Mage) have a limited selection as far as their magic damage reduction gear selection goes.

By no means is that a bad thing though, because in reality, magic defense bonus has relatively weak returns until you're stacking quite a bit of it, on top of obviously hitting the magic damage reduction cap itself beforehand.