I don't want to brag, but if I'm an expert in any area in BG2, it has to be mages. My party consists of FOUR mages, so I've had plenty of experience keeping them alive

Like everyone else said, keep them in the back. Also use pause a lot when spellcasting. What I disagree on is the use of magic missles and ranged weapons. I have three words, "MELF'S MINUTE METEOR'S." It was a kick-ass spell at the beginning of the game, and it continues to be as far as I've gotten (I'm at the end of Ch. 6). Memorize three or four per mage so they never run out. These babies do greater damage per hit than any ranged weapon, you can shoot five per round (wow!), and therefore are better than ranged weapons or magic missles (you can fire only up to five magic missles per round, and they do much less damage per missle than a meteor).
Here's another little tip: they have no duration limit, so cast one before you sleep and you will wake up with them already active, but still have all of them memorized. The same thing applies to stoneskin/ironskins, but it lasts only twelve hours, so you have four hours of free stoneskin/ironskins.
Now that I've covered typical combat, now for the spells! With the wide variety to choose from, innumerable strategies can be used. By far the best spell, that can be obtained in the game without the exp cap remover IMO, is Abil-Dhalzim's Horrid Wilting, which you get the ability to cast at 16th level. It does about 60-80 damage if no save, and 30-40 if save. In addition, it has a pretty good area affect, and DOES NOT harm your party members, or even innocent bystanders! I learned about this spell the hard way from that nasty lich in that storage container in the Underdark. By the way, an aerial servant (cleric spell level 6) kicks an invisible stalker's ass. How do I know this? I stage occasional *mock* battles for my personal amusement, as well as for information. Aerial servant is the best summoning spell for clerics, and animate dead (at least 15th level) is the best for mages (5th level spell). In combat, the aerial servant beats the skeleton warrior, but not as badly as it beats everything else.
Lastly, I haven't had a chance to verify this, but Mordenkainen's Sword (7th level mage spell) may be even better than aerial servant. In mock battles I have found the summoned flying sword to be impervious to ALL weapon damage, but it is extremely susceptible to magic, and even a couple of magic missles will kill it. But they could slice and dice anything that doesn't use magic, and hit as +4 weapons! I have yet to test it in true battle, so I don't know how good they truly are.
And if you truly want to know magical power, disable the experience cap with a utility at [url="http://www.teambg.com"]http://www.teambg.com[/url] so you can get 9th level spells. These spells are the ones often used by liches and Irenicus. I don't consider it cheating because you still have to earn the experience.
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Magus, mighty sorcerer, and summoner of hordes of...magical flying swords