Type: Defense
Strategy: Giving a 2 percent magic resistance per level to the target, this spell will bestow an exceptional magic resistance bonus to the target. One good strategy is to cast this spell on a fighter that already has a good magic resistance. With this spell cast, he will be nearly untouchable. Then, send him right into a group of enemies and wait for them to cast all their major spells (which are always cast first). Then, run him back to the rest of the group and slaughter your enemies. If that's too much to do, then the spell isn't great. With only one creature affected, it is impractical, like so many others.
Importance: Moderate
Type: Cure, heal
Strategy: Healing all party members hit points 1d8+1 per level, this spell is great during the middle of a battle when the whole party is hurt. Cast it, and everyone will recover a good amount of hit points. In the end, it will save your party and will keep all your party members in good shape.
Importance: High
Type: Imbuement - druid only
Strategy: Like a poor man's mass invisibility, pixie dust will make all creatures in an area invisible. The invisibility line of spells is one of the best set of utility spells in the game. When any of your party members is getting low on health, cast pixie dust to take him out of harm's way. If your mage is not needed at that moment, make him invisible, so he won't take unneeded hits. Lastly, if you must scout out an area for enemy strength and numbers, this is the answer to your problems. In this case, the spell will help the whole group.
Importance: High
Type: Cure, heal
Strategy: This spell is simple in use, but tremendous in power. When one of your characters dies, cast raise dead on him, and he will come back to life. Be careful, though, as the character will have no hit points when he reappears.
Importance: Vital
Type: Other
Strategy: Working like the clerical ability ward undead, repulse undead will force all undead away from the caster for one turn. No saving throw is allowed. However, where ward undead repeats infinitely and has a chance at destroying the undead outright, repulse undead is a one-time deal. The bonus of not allowing a saving throw is not worth using the spell over the limitless ability. Only use this spell if you are overwhelmed by the undead and need a moment's respite.
Importance: Low
Type: Imbuement
Strategy: The ultimate in the line of spells that increase the combat abilities of priests, righteous magic gives the caster one extra point of strength for every three levels. Also, the spell grants one additional hit point per level. Righteous magic will downright turn your priest into a godlike weapon of destruction. Adding as much as 24 points of strength, it literally turns your priest into a behemoth of mythic proportions and should be used whenever your priest must get his hands dirty in combat.
Importance: Vital
Type: Touch damage, summoning
Strategy: This spell summons a weapon with which the priest has 18 seconds to hit the target. If he does, the weapon will deal 2d6+9 points of damage and will force the enemy to make a save versus spells or be killed. The damage is great, but the ability to use this spell will rarely get past the save, though.
Importance: Moderate
Type: Dispel
Strategy: Like the mage spell true sight, this spell has the effect of dispelling all illusions in the area and can be useful if your enemy is a mage and has cast mislead or another beguiling spell. Otherwise, there isn't much of a use for it.
Importance: Low