Some of the more powerful creatures (okay, most of them...) in Baldur's Gate have special attacks that can bbe very disabling. The foul beasts and special spells to use against them are listed here in alphabetical order.

Ankhegs

The easy way to deal with Ankhegs is to have your fighter rush up and get within melee range. That generally stops the Ankhegs from casting their extremely nasty spit at your party members. You'll need a fighter with under zero AC (the lower the better) and preferably one with 30+ hit points.

An even easier method is to wait until it appears, then cast Sleep at it or at the ground near where it appears. The poor beastie will fall asleep most of the time at which point your party can spend a few rounds beating it to death with melee/ranged attacks. Note that Dynaheir cannot learn the Sleep spell due to her specialization, so take Xzar or Imoen (after dual-classing her) or try using Color Spray.

Color Spray (Wizard Spells, Level 1)

If you can manage to use this correctly, it's a powerful tool in your wizard's arsenal. Dynaheir, in particular, is limited to this form of crowd control due to her mage specialization. The important things to remember are that it's a cone shaped area of effect, aimed at wherever you click on the ground, and that it has a limited range and only affects up to 1d6 creatures.

Note: If you have party members within the area of effect, they must also make a saving throw vs spells to avoid being put to sleep for about 1 turn.

Against melee-only enemies, just put your mage in front and have them cast Color Spray towards the enemy before your fighters engage. Against ranged enemies, especially those that shoot poisons, send in a sacrificial fighter to absorb the hail of missiles and have your mage flank before using the spell. If you aim correctly, you can sometimes miss your fighter and only affect the enemies. But it's best to have a second fighter at hand to take over if the first fighter gets put to sleep as well.

A wizard's Sleep spell or a priest's Command spell are far easier to use.

Flesh Golems

Flesh Golems really aren't that difficult to deal with as long as you remember that they are immune to normal weapons and normal missiles. Get a fighter class with at least 35-40 hit points and an AC below zero up in their face, then have your other party members hit it with any magical weapon that they have. Bow, sling, crossbow and thrown missile users will need to switch to a magical melee weapon or use magical ammo. The major issue with Flesh Golems is that they hit like a truck, but really don't have too many hit points.

Ghouls and Ghasts

Ghouls are far easier then Ghasts, but both can freeze you in place (by casting Ghoul Touch) if they touch you. Once frozen, your character cannot move and all hits by the enemies will land. Which means extra healing required, but usually results in death(s). An early tactic is to outfit everyone in the party with ranged weapons, then have one of the characters play bait and run the undead in circles around those with ranged weapons.



A second option is Protection from Undead?

Sirines

The sirines (sirens) along the coast like to cast Dire Charm. Which means that members of your party will end up mind-controlled. You'll either have to spend valuable time casting Hold Person on those members (or Sleep, Color Spray, etc) or figure out how to avoid being charmed in the first place. Other tactics include Potion of Magic Blocking (which last 50 rounds or 5 turns) or scrolls of Protection from Magic (100 rounds). But the scrolls are extremely rare, so you'll need to look for potions instead.

I've had good success with a cleric's Command spell. Hold Person and Sleep are other options as well as Color Spray.



Undead - Using Turn Undead

Here are the gory details for how Turn Undead works. A level 3 cleric has about a 50% chance of turning Ghouls, but only a 10% chance of turning Ghasts. Which are the big two that you'll find in early questing that can give a low-level party a lot of trouble.

At lower-levels, Turn Undead is more of a annoyance then a help, because while the undead will turn and run away, they will also run away (or shamble away) which means you have to chase them down. That can lead to a cascading series of encounters in areas where you haven't cleared everything. Until your cleric gets to level 8-10, you won't see much offensive use out of Turn Undead (where the undead are destroyed). I suggest not using Turn Undead against skeletons, which move quickly and save it for Ghouls/Ghasts which just shamble around slowly. That way, when a Ghoul/Ghast gets turned and runs away, it won't get as far as fast before you can kill it.