I have beat the game a few times with solo chars now, of which a few were not totally solo because I could resist bringing Imoen along from a role playing aspect.
The hardest was with a solo Druid Fighter. Quite difficult, and, yet in some parts, to easy. He used the Dagger of Venom and the +1 sling. Mages were stabbed and the poison would prevent spell casting. Animal summonings took care of most mobs. No Imoen this go round. Getting past those skeleton archers in the maze was down right frusterating. Awful really.
I had a Fighter Mage Thief who took Imoen along. He had it quite easy for most parts, used that Cloudkill scroll to make the final battle a whole lot easier. Composite Long Bow for most of the game made it to easy. Cast sleep, shoot, cast blind, hold person, etc, shoot.
The solo mage could not get past the Doppleganger fight in the Duchal Palace no matter how hard I tried, so, he hung up his robes. I could not, no matter how hard I tried, keep one of those silly nobles alive. He was a pure mage by the way, no specialty. I figured every spell would be vital. BTW, in my version of BG on the Mac, the Ring of Wizardry is totally disabled, you can't get one from the FAI, but, you can get one near the end of the game. His most used spell was Larloch's Minor Drain oddly enough. He would use it to seriously buff up his hit points duing battle. Also, it was very quick to cast so it was used to interupt enemy mages.
The solo cleric kept dying badly. He could not survive the fight in the whorehouse down in the sewers of BG.
My greatist victory though was taking a Bard through the game solo. I dunno how I did that, mostly through the use of archery, charm, sleep, and blindness spells. Strangely, the Bard had a very high survival rate. He used hundreds of Arrows of Biting as well as any other sort of magical ammo. He used both bows and crossbows. The crossbow of speed along with acid bolts is a very deadly combination along with Haste, the Boots of Speed, etc. Most fights were just avoided later after I hit the exp cap, which I did so very very quickly. The final fight was done with that enchanted throwing axe and a bunch of spells, along with dozens of summoned monsters. Bardsong seems to effect summons. I noticed when he sang, his summons lasted a long longer, resisted deadly effects, hit more often, seemed to take less damage, etc.