Type: Missile damage
Strategy: This is a fire-based flamethrower spell that will do three to 18 points of damage to any enemy. The nice thing is that there is no save. But on the downside, the jet will stay for a round or two, and anything walking into it will be burned, party members included. This is a good spell to cast when monsters are lined up. Just cast it on the one furthest away, and all the monsters will take the damage. The spell does not gain potency upon leveling up, so it will lose its steam as the game progresses.
Importance: Moderate
Type: Defense
Strategy: Lowering the caster's armor class by three regardless, this is a very good armor spell. The armor-class bonus will greatly help your mage's resilience and will let him stand up to much more of a beating. If you think you will be in an area where your mage will come under attack, casting this spell will greatly lower the damage he takes.
Importance: High
Type: Hindrance
Strategy: Forcing a 50 percent chance to miscast spells on the recipients, this spell is actually not useful. Though spells that hinder enemy casters are always a help, it is a much better use of your time to use silence, as the recipients lose all ability to cast spells. In the end, silence does a lot more a lot faster.
Importance: Low
Type: Other
Strategy: If your party ever comes under attack from any type of invisible creature, casting this spell will force it back into visibility. However, on your journey, you will rarely face invisible creatures, and when you do, they will become barely visible when they attack you. Unless they have used greater invisibility, you can attack them (with penalties). Also, you can easily cast dispel to get rid of the invisibility if needed.
Importance: Low
Type: Touch damage
Strategy: Though the effect of paralysis is not shabby, to reap its rewards, you must not only move your mage into combat, you must hit the target creature with a melee weapon. Mages usually have a difficult time hitting, plus the target gets a saving throw versus the spell, so the spell is nearly useless alone. If you prepare your mage with good defensive spells like blur and mirror image, though, the effects of entering combat are not as bad. The paralysis effect is stellar when it isn't resisted, so it can be worth your effort to give it a try from time to time. When it works, it works well.
Importance: Moderate
Type: Hindrance, other
Strategy: Though the side effect of revealing invisible creatures is nice, most of the time, the main blinding ability of the spell is resisted. Though this does reveal invisible creatures and should automatically take a place above detect invisibility, it is still nearly useless. Just use dispel for the invisibility and a more potent spell for everything else.
Importance: Low
Type: Hindrance
Strategy: This is a nice spell if you are getting overwhelmed by low-level enemies and need time to deal with a few of the enemies at a time. Horror instills fear in it targets and will give you time to hit them when they can't hit back. Remember, though, that when the spell wears off, the feared creatures will be back in action. The monsters get a save, so do not expect them all to be affected. However, there are higher-level fear spells that can take effect on high-level creatures, get resisted less, and do more. For the level, though, the spell works acceptably on those lower-level creatures.
Importance: Moderate
Type: Other
Strategy: The invisibility line of spells is one of the best set of utility spells in the game. When any of your party members are getting low on health, cast invisibility to take him out of harm's way. If your mage is not needed at that moment, cast invisibility on him, so he won't take unneeded hits. Lastly, if you must scout out an area for enemy strength and numbers, this will answer your problems.
Importance: High
Type: Other
Strategy: If you ever find yourself without a good thief, or if you find a lock your thief can't pick (assuming it is available and not barred or magically warded - the game will tell you if it is), this spell will open it. If you do have a thief that is proficient at lock picking, this spell will not help.
Importance: Low
Type: Other
Strategy: If you ever find yourself in a situation where you are not sure whether to trust or distrust what you are dealing with, casting this spell will help you. However, it is usually more beneficial to follow through and trust, even if you know it's a trap, as the result can lead to hefty treasure, experience, and even new party members.
Importance: Low