Well i have just finished the game with a party of one. I used a warrior/thief/mage half-elf. With the thief part, you can hide in the dark and sneak past monsters or backstab them. The warrior part allows you to use all the weapons you can find. And some mage's spells help a lot to solo all the creatures you can find.
When you max your exp, you get a level 10 warrior,11 mage and 12 thief. I must have around 100HP, Ac -
with a very nice mage robe that i got from one of the last chapter "boss", deep in the dwarf town), TACO 4 (with a +3 scimitar).
By the way i'm french and i have the french version of the game, so i'm sorry if don't know the english name of the spells i use.
So for level one spells, i use a direct damage spell (mainly the one that throw red balls .. sorry I dont even remember it's name in french)because it is very fast to cast and can interrupt casters, and one or two identify spells.
For level two spells, i only use the spells that makes duplicate images of you so the creature must first "kill" them before they can touch you. A very useful spell, as it can also protect you from spells...
For level three spells, i only use "haste", makes you run and hit faster, very useful also if you have several creatures to kill.
For level 4 and up, the spells I choose depend on the situation. Greater invisibility can be useful if you dont want to hide in the dark all the time. Area spells like "death cloud" can be useful if you want to clean a room full of nasty little critters (especially archers). I didnt find a lot of use for the invocation spells, too few creatures and sometimes to weak to really help in the later stages. Strong damage spells can also be useful.
Useally i cast haste, image reflection(?), hide and go explore the zone. I try to backstab a creature when i can. By the way, i always use a very fast weapon (0 to 2 for speed). As long as i dont get surrounded too much, no problem, i usually can solo a monster easily.
The most annoying creatures are archers, as they can dispell my reflections while i fight another creature. I usually go for them first, either with haste (and with my boot of speed later on) or with spells.
In Icewind Dale, i found the used the thief part a little less useful than in Baldur's gate (i also finished baldur's gate with only a war/mag/thief). In Baldur's gate, i think i needed more detect trap and hide in shadows, as i think there were more traps on the ground. I'm not sure i could have finished BG with only a warrior/mage player (or it would have been much harder). In Icewind Dale, i think a warrior/mage can make it easier. I used less backstab in Icewind Dale and never looked for traps on the ground, only on a few chests when i remembered to do it...Anyway spells protect me.
And in BG, i used a lot of summuning creatures staffs. I really needed them to survive and kill all the monsters (like with the dopplegangers episode in Baldur's gate when you must protect two people from being killed by them). For the final boss in BG, i drowned him and his friends under creatures and i just delivered the killing blow with a nice backstab.
Well i have never used a single summoning creature staff in Icewind Dale... for the good reason that i didn't find one... Well i didn't need it anyway, i just cast a few summoning spells one or two times, but i didn't really feel the need for it.
The only "hard" part was with the final "boss", i summoned some creatures to distract it a little while i attacked him "hand to hand". By the way, i managed to kill it with the help of a "thunderbold staff", because i was a bit short on spells and without the help of haste and image reflection, i can't hope to win in close combat against him.
Well, anyway i think it is a nice challenge to try to beat the game with only a party of one. And it can even easier in a certain kind of way, as you only have one player to command...
Ah, by the way, maybe the biggest problem you will face is with what you can carry... You can't get all the nice items you will find with only one player... Either throw them or make a lot of trips to sell them.
One last point on my equipment... I only used mage's robe as i didn't wait to start pausing the game to cast my spells, change armor to fight and so on. I found a ring that double the number of level 1 and 2 spells i can memorize. Another ring gives +1 in stenght (doesnt work by the way, it changes my 18/85 to 18/91) and another attack to a thief only. I have a pair of boot of speed. Either a +3 ice scimitar or a +2 scimitar with another attack. My bracers give my -1 to TACO. And i dont remember the rest, it is the pretty classical magical stuff you wil find.
Good luck to you