Type: Summoning
Strategy: Summoning a +3 magical sword (that your mage can use) that also does +10 extra damage versus the undead, this spell is good if you plan on sending your mage into combat. Summon the blade and defend your mage with armor spells, and he will turn into a virtual fighter. However, keeping your mages back is usually the better strategy, as it is usually better to take advantage of their ultra-useful spells. That is what they're there for now, isn't it?
Importance: Moderate
Type: Defense
Strategy: Working like all other protection spells, protection from acid confers immunity to all acid damage. It's useful if you are up against a mage who likes Melf's acid arrow, but it's usually of no help.
Importance: Low
Type: Defense
Strategy: Working like all other protection spells, protection from electricity confers immunity to all electric damage. It's useful if you are up against a mage who likes lightning bolt, or if you find yourself in an area with lightning traps. Usually, it's of no help at all.
Importance: Low
Type: Defense
Strategy: This spell is useful if your mage is hurt. With this spell, the caster will be whisked away out of view for the duration of the spell. It works much like invisibility, but with a key difference: The only means of dispelling this spell is to use a true sight spell. However, invisibility works just as well as a getaway spell and is available at a much lower level.
Importance: Low
Type: Defense
Strategy: Rendering the caster immune to all spells of a specific school, spell immunity works wonders once you know what your enemies are casting. Usually, a choice of evocation works best (though the circumstances vary highly) and will shield you from most damage spells. It is definitely advised that, once you know what the enemy is casting, you should look the spell up in game or in the manual and see what school it is. From there, you can make an educated choice about what school to use for spell immunity.
Importance: High
Type: Defense
Strategy: Creating a shield that prevents one dispel-type spell from reaching the caster, this spell can be as much a hindrance as a help. If any other type of detrimental spell is cast on you, it will be impossible to dispel that before dispelling this anyway. So, unless your mage is extraordinarily well-protected, it is advised to keep away from this spell.
Importance: Low
Type: Area damage
Strategy: Sunfire works exactly the same as fireball, with the exception of one important fact. Sunfire will not harm the caster. However, since sunfire still harms the party, it is difficult to use if your mage is not alone. But when your mage is alone, it is very easy to obliterate a group of enemies that are surrounding him. In essence turning your mage into a grenade, this spell opens up totally new scenarios for fighting packs of enemies. After some trial and error, sunfire works wonders.
Importance: High