Below are some strategies that I use when trying to get through difficult areas.
Not only should you save your game after every battle or major dialogue, I also recommend keeping several saved game files for each of your games. This will allow to backtrack several saves if you, for instance, find yourself in an area that is too dificult or you don't like how a subquest you are involved with is turning out. I usually have one save game from the beginning of the current chapter, one save game from the last safe area (e.g town) I was in, one save game from the beginning of the current area I'm working on, and then one or more in the current area. I often make temporary saves if I want to experiment somewhere but don't want my experiments as part of my main game.
Ranged Attacks Rock!
I usually find ranged attacks useless in most RPGs, but in Baldur's Gate they are quite powerful. I recommend that most if not all your fighters have some sort of ranged attack available. You can usually take out quite a few of the enemy before they come into melee range. Ranged weapons are also useful in taking out enemies one at a time as I will explain in the next hint.
Take out enemies one at a time
A strategy I use for nearly all difficult battles I encounter is to take out the enemies one at a time. As you approach the area where the enemies are, take one step at a time. When a single enemy appears, stop and take him out either with ranged weapons or with melee weapons when it reaches your party. Once the enemy is dead, take another step forward until the next enemy appears. This way you will take out each of the enemies in the group six-on-one instead of being outnumbered. You can even save between enemies. This works well against normal enemies and NPC parties.
My Favorite Spells
By far my favorite spell, I use it whenever I want to kick some major enemy butt. This devastating spell can wipe out an entire group of weaker enemies and cause serious damage to more powerful opponents. Collect as many Wands of Fire as you can, since they have the same effect as the Fireball spell.
I find this powerful spell almost as indispensable as the Fireball spell. Cast this on a group on enemy NPCs to remove all of their invisibilty, blur, mirror image, ghost armor, etc effects and cast it on your own party to remove all confusion, fear, and hold effects.
Cure Light Wounds
I usually have my clerics use this spell to fill all of their first level spell slots. Since you only heal by about two or three hit points apiece when you rest, you need to have healing magic to fully restore your hit points. How many Cure Light Wounds spells you have available basically determines how long you can adventure before resting.
This spell has several advantages:
I like using this spell agains enemy mages because it disrupts any spell they are casting (since it is a guaranteed hit) and it usually only takes one or two spells to kill them. It is also useful to help your fighters defeat enemy fighers.
Cast this spell on a group of enemies and then take them out using ranged attacks from outside the area of effect. This is the best area spell until you learn Fireball.
This fifth level spell is incredible powerful. The catch is that I was only able to find one of these and it cannot be memorized from your spell book (you can only memorize up to level 4 spells in Baldur's Gate). I used this spell in the final battle and was quite pleased with the results.
Draw Upon Holy Might
This spell gives the cleric a significant advantage in battle. The hero learns this in one of the later chapters.
Don't waste your gold identifying items in shops; identify them yourself for free.
Protection from Petrification
Cast this on one of your characters and they are almost invincible against basilisks.