Contingency and Backstabbing
Approach the mage unnoticed, either with invisibility or him waiting for a dialog.

1) Contingency a few Magic Missiles (the game's best spell!!!) My three mages unleashed 5 Magic Missle spells (2 mages with contingency, 1 mage with standard magic missiles), he did not have a chance!

2) Powerup a backstabber (Valygar the ranger works nice) and then make him invisible, walk in an backstab.

Note: Improved invisbility is better because the mage cannot target you if he is still living.

This works great as most powerful mages have contingencys ready to release their Protection from God spells. Often the mage of the group will kill you by casting Confusion or Chaos and allowing his fighters kill you or by casting continued spells at you.

Submitted By: Jason

Give Them the Plague
One very effective tactic against caster heavy opponents (wizards, clerics, druids) is the druid spell insect plague. Cast this spell on an opponent, and it'll spread to 5 others within a certain radius. The spell will reduce AC, damage for 1 hp per round, and penalize a caster with 100% spell failure.

As most caster opponents will use the initial round to raise their defences, this will give u time to cast insect plague on them. This will effectively make them useless for the remaining rounds as they cast spell after spell only to fail. Use this time to wipe out any fighter support the wizard has, and to breach the wizard's defenses. This will also prevent those annoying charm, chaos and confusion spells from being cast on your party.

Submitted By: Eugene

If They're Immune to the Plague?
Many people overlook the fact that the spell "lower resistance" is considered as an attack in BG2. This means every time you see them start casting, cast lower resistance and you'll see them fail their spell. A funny bug, but a sure way to disable any spellcaster. Since lower resistance only takes a few seconds to cast (faster if you have the Robe of Vecna), it'll hit before the hostile spellcaster gets his spells out. Also, lower resistance will not be affected by spell turning/deflection/trap, (minor) globe of invulnerability, and better yet, magic resistance. Of course, this is when you're up against a single or solo powerful spellcaster who is immune to insect plague. Else, insect plague works fine on the weaker spellcasters.

I've tried this on Liches, Irenicus and other powerful spellcasters and it works great. This also works on stopping dragons from casting spells.

Submitted By: Sleepy

High Level Animation
The spell "Animate Dead" works great against most spellcasters, at least when you've reached level 15. Just stand where the wizard can't see you, summon a skeleton and send it towards the enemy. He will then try to target the skeleton with all sorts of spells, but as skeleton warriors have very high magic resistance almost all of them will have no effect. Just wait until the enemy mage has wasted all his spells and his contingencies have worn off and go cut him to pieces.

NOTE: I haven't tried this with the skeletons you get before lvl 15 yet, so I'm not sure if they work.

Submitted By: Hannwaas