[SixOfSpades]

It is very, VERY important that you learn how to fight Mages. Don't put a lot of faith in the fact that you're a Wizard Slayer, they pretty much suck at killing spellcasters. You don't believe me, try playing with an Inquisitor in the party and you'll see who really gets the job done. But I digress.

There are many ways of dealing with spellcasters:
1) Break down their buffing and protection spells. Popular ways to do this include Breach, Remove Magic, Pierce Shield, Ruby Ray of Reversal, etc., as well as simply running away (or hiding) and waiting for their spells to expire. Invisibility spells are included in this group as well, and great ways to reveal enemy wizards include True Sight and a Thief's Detect Illusions skill.

2) Become temporarily immune to their magics. The important things to watch out for are Fire damage, Magic damage, Hold/Stun effects, and mind-affecting spells. Most of these can be countered both through spells and potions: For example, if you want to resist Fire, you can drink 1 or 2 Potions of Fire Resistance, and/or have a Wizard or Priest cast Protection from Fire on you, a Wizard could also cast Protection from the Elements or Protection from Energy, a Priest could cast Resist Fire/Cold, somebody with at least 9 INT could read a Protection from Fire scroll, or if your timing is very good, drink a Potion of Icedust. Note that all of these means, however, can be dispelled as I mentioned above in 1).

3) Become permanently immune to their magics. This can be accomplished only though innate immunities or resistances, such as a Cavalier's permanent immunity to all Poison damage, or through items, such as the +40% Fire Resistance granted by the Ring of Fire Resistance that was in BG1. On the whole, these resistances and immunities cannot be dispelled the way buffing spells can, but there are exceptions: Your Magic Resistance can be lowered by Wizards targeting spells such as Lower Resistance at you, and sometimes Dragons can cast an area-of-effect spell that makes those affected by it become more vulnerable to the Dragon's own element--a Brown Dragon, for instance, might cast Lower Acid Resistance at you before using its Brown Dragon Acid Breath.

4) Kill 'em before they can get any good spells off. Ways to do this involve fast, usually area-of-effect spells; they don't have to do a whole lot of damage, since Mages generally don't have a whole lot of hitpoints. Of course, this won't work too well if they've got their buffing spells up: It doesn't matter how many Fireballs you cast at a Mage if he's already under Protection from Fire.

5) Incapacitate 'em before they can get any good spells off. This is usually more effective than 4), because most Mages protect themselves against Physical and Elemental damage, not Hold/Stun effects or mind-affecting spells. So Web, Stinking Cloud, Greater Command, Emotion: Hopelessness, and of course the big gun as far as anti-spellcaster tricks is concerned, Insect Plague.