Hmm... I did solo run of IWD2 and while I gained not so much XP from killling single monster since it was not divided amongst 6 members I levelled fast enough to deal with encounters.
What was the character you used for your solo run? I tried to solo the game at first, but it was just too brutal. The game kept throwing at me huge number of enemies - much more then a full 6 character party would ever fight because of its broken scaling system. I finally settled for a two character party and got quite far before giving up in disgust.
But anyway, I'm still against scaling of xp rewards by character level. I agree that an experienced fighter should not get as much from fighting weaker opponents compared to an inexperienced one, but there is already a much more elegant way to ensure that - increase of the amount of experience points needed to achieve the next level. An experienced high level fighter would need much more experience to get to the next level and will need to defeat many, many weaker opponents to do so. The low level fighter will on the other hand level-up quickly even by fighting just those weaker opponents. This is already so in DnD and similar systems and further scaling of experience you get by defeating enemies is unnecessary balancing that just complicates things and gives the player the feeling that he is being punished for doing too well.
My approach was quite tricky and required some meta-gaming knowledge.
I decided that cleric was a way to go as a main class since it’s very universal class in IWD2 - quite good melee abilities, heavy armor and spell casting and you still can learn some thieving abilities. And buffed cleric has nearly the same combat stats as pure fighter (except number of attacks and bonus fighter feats) further closing the gap. Since good cleric has spontaneous casting of healing spells you can memorize only (de)buffs/offensive spells and still be able to heal yourself on the fly.
As for the race I have chosen drow. +2 levels penalty is not a problem when you play solo and gain loads of XP, you get some bonus attribute upgrades, spell resistance (for me it’s drow’s killer feature, especially that it’s rising with character level), immunity to Charm/Sleep, darkvision and some minor bonuses. -2 to hit/damage in sunlight is noticeable in the beginning but ones you gain some levels and get enough buffing spells even fighting in the sunlight is not so terrible. And you may wait until dawn to go hunting outside :> And guess what? Female drow has cleric as favoured class!
On the other hand it’s good to finally gain access to some serious weapon and the ultimate choice is Holy Avenger (*): +5 sword, +2d6 bonus damage to evil, unlimited dispel magic and last but not least +15 spell resistance. So a drow of character level 15 with holy avenger has 11+15+15 = 41 spell resistance just like that! It’s one handed sword, so you can still have a shield! Too bad it’s usable only by paladins… But if you make your female drow lawfull good and she is Painbearer of Ilmater then she can multi class to Paladin of Ilmater and advance in both classes! And if you invest some points in charisma then being paladin gives you bonuses to saving throws, and even 2 leveles of paladin gives you immunity to diseases and fear.
Palladin of Ilmater (2)/Painbearer of Ilmater (17) = 19 character level, not a problem solo – 19 is reachable with party of 6 I think. 17th level of cleric gives you access to one 9th level spell slot, and each next gives you one more until level 20 which gives 4 slots for 9th level spells. Anyway, if you boost wisdom really, really high it’s not necessary to advance beyond level 17 as Cleric (you already have access to 9th level spells) since you start to get bonus 9th level spells because of your Wisdom. But AFAIR it’s something around score equal 30. Remaining levels go into Paladin. Theoretically you can also start as Paladin until level 3, switch to fighter and gain 4 levels to get some bonus combat feats and weapon specialization to maximize effectiveness of Holy Avenger (+1 to hit/+2damage) and after that go with cleric. Please have in mind that you temporarily have XP penalty when leveling fighter until he reaches level 2. But it’s not a huge problem since at the beginning of the game you gain levels at lightning speed anyway. And since cleric is favored class of female drows, this does not count. So this build would look like this:
Palladin of Ilmater (3)/Fighter (4 levels)/Cleric - Painbearer of Ilmater (17/20)
So the character level required for a full build is 3+4+17=24/27 (+2 drow penalty), so full potential (high level cleric spells) is achieved very, very late game or even not at all. So I would stay with Palladin of Ilmater (2+)/Painbearer of Ilmater (20).
Anyway, game is beatable with such character, but not an Iron Man mode really. I had to reload a lot! Maybe there are some better builds for solo but I was so proud when I analyzed the IWD2 manual on my own, discovered all theses synergies and was able to finish the game with my own build. But feel free to dig the internet – some people have finished solo with a pure Sorcerer (can’t imagine this!) or some WTF hybrids like Sorcer/Barbarian...