I dont know about the Dos version but on the NES and Origins version the solo thief is much harder. Although the Bm has lower hp and attack, he still has magic spells to kill groups of enemies in 1 turn, unlike the thief. His defense is just as good as the thiefs at the end too.
Oh and the solo Black Belt isnt hard at all. It is actually one of the easiest solo games you can do.