Kra'an has fallen to my solo Necromancer. It was very enjoyable and
a good challenge to play through the game with a solo character. I
picked the Necromancer ("Ensir") because he is self-healing, can
summon help (Wind Mage or Fire Elemental), and has some decent
offensive spells.

Tips and Observations:

My favorite Necromancer spell is "Life Vortex", as it replenishes
both hit points and spell points but doesn't use as many spell points
as "Death's Blessing". I found that I really welcomed large groups
like 3 banshees or 5 lizard warriors because I could cast repeated
Life Vortex spells and completely recharge my character.

While all adventuring was done with the solo character, I did pull
the Thief out of the inn and walk him around now and then for some
cash. At the lower levels, gathering funds to buy things is a real
challenge.

I found that the lack of Levitate and other protective spells was not
a problem, but the lack of LIGHT was a real pain. With a solo
character, you have so few inventory slots, and carrying a torch
around wastes a valuable slot. I was very happy to finally get a
glowing double crossbow* and leave the torches behind.

The real secret to soloing Raamtha and Kra'an was building a
sufficiently large character, and the secret to that was one word:
zombies. That room in the crypt with the re-appearing zombies
provides a fantastic means to build up a character to absurd
heights. Just don't kill the Zombie Lord, but keep knocking off the
pop-up zombies for 2100 points apiece. You and your fire elemental
can kill one every two rounds. When I finally killed off the Zombie
Lord, I had a 24th level Necromancer with 300+ hitpoints and almost
500 spell points.

I found Kra'an easier to kill than Raamtha, because with Raamtha
casting Heal Party for 35 points a round, the monks were a fairly
tough bunch kill off. Also, when fighting Raamtha, there is no good
hand weapon for a Necromancer, and the double crossbow took a looong
time to drop him - again, because of his constant healing. Kra'an
was much easier, as I could Dispell the demons and charge in with the
Dragon Bane sword. By constantly charging, Kra'an has less time to
heal himself. I used all the strength-enhancing items I could to
maximize damage per swing.

So Dwarf and Thief are not the only good solo characters. I'm not
sure what the hardest would be, but I think adventuring without the
ability to heal would be tedious.

Good game - thanks Mythological!