levelworm: Alright, so here is my short non-spoiler mage battle tips for BG2, I can also write one for BG1 if you are interested.
1. You need to protect your team against Fear/Confusion/Charm/Domination, all those spells that skew your mind. There are a few items that negate such spells, say, The Equalizer the Two-Handed Sword, or, Helm of Charm Protection. I'm not going to make a list here, as you can read the descriptions yourself. We must keep in mind, however, that such items are rare and only protect the wearer from certain spells, for example, the Helm of Charm Protection will not protect its wearer against Confusion. Fortunately, you also have counter-spells. Resist Fear and Chaotic Command are two valuable spells as the first counter Fear or similar effects while the second protects against virtually every mind-skrewing spells. Note that some innate abilities, such as Berserk, also work.
2. You also need some protection from Level Drain when dealing with vampires. There are a few items that deal with it but I'm going to tell you that the spell Restoration removes the Level Drain and similar effects, say, Energy Drain and Ability Drain. The spell Lesser Restoration has NO effect on Level Drain, but if my memory does not fail me, it should at least remove Energy Drain. BTW sleeping won't remove Level Drain, which makes it more dreadful.
3. Besides of the mind-skrewing spells, you also need to be aware of the paralyzing spells or similar effects. If your characters are under the influence of Hold/Paralyze, they are plainly dead. The spell Remove Paralyze is great under this circumstance. Additionally, any item that gives you Free Action protects you from being Held/Paralyzed. I'm not sure if these items are powerful enough to counter the most powerful foes, say, Balors, but you should be OK most of the time.
4. There are also other nasty effects, say, Maze or Imprisonment, but you won't see them very often. Besides of all I talked above, there is another way to deal with these spells, i.e. to steel your Saving Throws against such effects. Leveling up definitely helps, but there are also items and spells that help, too. The simple spells of Chant and Bless raise Saving Throws by 1, read the description for detail.
5. Now, for your magical defense. Stoneskin and Ghost Armor (or the better version) are amongst the commonly used. But for defending against magical blasts, many other spells must be cast before/during the fight. Protection from Magical Energy is very useful in later chapters as it renders a single character immune to Horrid Witting. Mislead and Improved Invisibility renders the target with invisibility as well as other benefits, and remember that when you are invisible they cannot target you with magical spells. Of course you will still be influenced by area spells, but that's a different story. Spell Turning can reflect enemy spells (not the area ones, though), which makes them quite useful, what's better is that they are not effected by Dispel Magic. The 9th level magic Spell Trap is extremely powerful and I'll leave you to read the description. Protection from Magical Weapons can render you immune to most of the physical attacks as the enemies mostly use magical weapons in later chapters. The 6th level spell Contigency is good for emergencies, check the description for detail. There are other spells that are useful, say, Spell Shield/Spell Deflection, but I'm sure that you can find them by yourself.
I'm going to finish the Defensive part here, and continue the Offensive part in the next section.
Thanks, that's a good guide for buffing up the party. Some of the fights are just plain nasty, however, like mindflayers. At least in my experience, beating them is down to luck in who they target and whether they can get to them. Chaotic commands helps, but they can still drain you incredibly fast.
99% sure Lesser restoration (the level 4 or 5) type fixes level drain though (at least the type early-ish encounters with vampires and such cause). I've done it before with Aerie, without going to a temple. She gets fatigued, but the level drain is gone. Some other effects, like reduction in strength (ability drain I guess) are temporary and will wear off if you just wait a little.
Chant and Bless are spells I don't use enough. Low level spells too, and pretty handy, but when the shit hits the fan I tend to start with more immediate concerns, as you can only cast one spell (per character) per round.
The innate abilities of Keldorn to cast True Sight and Dispel Magic is also worth a lot. True Sight means spellcasters can focus on other spells, as Keldorn can mess up enemy mages' illusions on his own. And his Dispel Magic hits a lot harder than 'normal' Dispel Magic. Think his levels gets doubled for the effect. Very handy. Of course, your buffs go out too, but it may be worth it if in a bind.
Look forward to the offensive guide :)
(If you are writing several suchs posts, it may be worth to put in a new topic btw, as it can get 'lost' in here. I'm sure many will appreciate it, as it's a pretty tough thing to deal with for new players, such as myself.)