NulMgc

Type White Magic
Level 7
Effect Reinforces protection against instant death.
Target All allies
Price 45,000 gil
Sold In Onlak

Learned By

WM
RM
BM
Kn
WW
RW
BW
Ni
NulMgc purportedly grants resistance to KO Spells, but this description isn't entirely accurate. It does give Death elemental resistance, as well as Earth and Status resistance, but does nothing to stop the Time- or Poison-elemental KO Spells you will run into late in the game. Still, it's the only way to gain resistance to Earth or Status short of a Ribbon or the NulAll Spell, which can be nice. It's of limited use once you've found all three Ribbons, though. It is one of the two level 7 Spells Red Wizards can learn, though, so at least for them it's a no-brainer.
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Related

NulBolt

Type White Magic
Level 2
Effect Reduces lightning damage by 50%.
Target All allies
Price 400 gil
Sold In Pravoca

Learned By

WM
RM
BM
Kn
WW
RW
BW
Ni
The Nul Spells are all quite good, effectively decreasing damage from a given Element by a half to three-quarters, and NulBolt is no exception. However, this Spell isn't urgently needed because you're not liable to be hit with Lightning damage for quite some time. Astos could get around to casting Bolt2 if the battle wears on long enough, as can Lich, but the first time you're actually likely to get hit by it isn't until the Ice Cavern. So feel free to save some money and wait on this Spell, but don't forget it. At least, not until everyone in the party has gear granting them resistance to Lightning anyway.

NulFire

Type White Magic
Level 3
Effect Reduces fire damage by 50%.
Target All allies
Price 1,500 gil
Sold In Elfheim

Learned By

WM
RM
BM
Kn
WW
RW
BW
Ni
You won't run into any Fire-elemental attacks until you reach Mt. Gulg, with the possible exceptions of Fire2 Spells cast late in the battles against Astos or Lich, but when you do get there you'll be very happy you have NulFire. Resistance not only halves damage, but basically ensures the Spell or Special Attack will miss, dealing only a quarter of its maximum damage overall. And NulFire grants resistance to the entire party with a single casting. A great Spell.

NulIce

Type White Magic
Level 4
Effect Reduces ice damage by 50%.
Target All allies
Price 4,000 gil
Sold In Elfheim

Learned By

WM
RM
BM
Kn
WW
RW
BW
Ni
The only level 4 white magic Spell that is unquestionably good, NulIce is hardly a Spell you ever want to call the best of its level. It's great against White Dragons and Winter Wolves in the Ice Cavern, and is useful to use against Lich since his first Spell will always be Ice2. Those uses are important, but beyond that you're not likely to be casting NulIce much.

NulAll

Type White Magic
Level 8
Effect Reduces all spell damage by 50%.
Target One ally
Price 60,000 gil
Sold In Gaia

Learned By

WM
RM
BM
Kn
WW
RW
BW
Ni
NulAll is one of those Spells that sounds far better in description than it is in practice. Sure, it grants resistance to all eight Elements, but so do Ribbons, and you get three of those. Assuming you use them, this Spell can only enhance defense one party member, who may very well be a Knight that already resists the main attack Elements and Poison anyway. On the upside, at least being a single-target Spell doesn't make it any less useful in this case.

ARUB

Sprite
Type White Magic LV 7
Effect Grants all allies resistance to Status, Death, and Earth
Target All allies

Learned by

WM
RM
BM
Kn
WW
RW
BW
Ni
Sold In Onrac (45,000 G)
ARUB grants resistance to Death, but this isn't the whole story: it also grants Earth and Status resistance, though it does nothing to stop the Time- or Poison-elemental KO Spells you will run into late in the game. Still, it's the only way to gain resistance to Earth or Status short of a Ribbon or the WALL Spell. It is one of only two level 7 Spells RedWizzes can learn.