Might as well put this up for a reference point when I've discussed how bad Rosa is late game...
Rosa in FF2:

Equipment:
Life Staff
Silver Shield (shield trick)
Ribbon
Heroine Robe
Protect Ring
Rosa in FF4:

Equipment:
Artemis Bow + Artemis Arrows
Ribbon
White Robe
Protect Ring
Rosa in FF4ET:

Equipment:
Artemis Bow + Yoichi/Samurai Arrows (add 25 to attack power if Artemis Arrows are in use)
Ribbon (which has elemental absorb to the basic elementals)
Heroine Robe (which has bonuses of +10 STR, AGI, VIT instead)
Crystal Ring
FF4ET has the right idea as to what needed tweaking... Rosa is too weak/slow in comparison to the other characters...
Rydia in FF2/4:

Equipment:
Dragon Whip
Silver Shield (Shield Trick)
Ribbon
Sorceror/Light Robe
Protect Ring
See, even by Rydia's standards, Rosa barely keeps up with Rydia's attack power because she
must rely on getting the Artemis Bow.. which is very important to consider. Factor that the Artemis Bow is a rare drop, makes this a pain in the ass.
Edge in FF2/4:

Equipment:
Masamune
Murasame
Ribbon
Ninja Robe/Dark Gear
Protect Ring
It is arguable that Edge is supposed to have the worst magic defense (he does for a long time anyways). However, by comparison, he's actually on par, if not better than Rosa in these regards. His limited (and nonexistant) gain in Will Power limits his magic defense severely (like resistance to Sleep with the Ninja Hat will help him)
Edge in FF4ET:

Equipment:
Masamune
Murasame
Ribbon (elemental absorb ftw)
Ninja Robe/Dark Gear (+5 Agi instead of the usual +3 Agi)
Crystal Ring
Edge isn't losing too much in the magic defense department, but still just as good magic defensively.
Anyways, Rosa is the worst character in the group, and by a good margin. Anything less than an Artemis Bow or a Life or Silence Staff/Heroine Robe will prevent Rosa from reaching a 6x magic defense multiplier... that's not even factoring her less that desirable stat growth for Will Power.
Rosa has always been the best candidate for the Adamant Armor, and there's the proof as to why.
Edit: Here's another setup I've just considered...
Rosa:

Equipment:
Elven Bow + Artemis Arrows
Tiara (this means no status protection, but you have resistance vs Dragons)
Heroine Robe (you only get Paralysis protection, which is very useful)
Protect Ring
You get the magic multiplier... have much less status protection (which may not really matter at this point) and pretty much declare that the Elven Bow is the most useful of the bows if you don't have the Artemis Bow (if Yoichi's Bow had a different stat boost, it would be different).