I agree with Stingray. In all games I know bosses are significantly more difficult than regular enemies. ADOM pre-p9 is actually the same: Kranach, Keethrax, Orb guardians and other such bosses are harder than any single enemy on their respective dungeon levels. This should be the case with boss monsters too, or, if that goes against the idea of any particular boss, they shouldn't be generated where they could end up below average at best.
After all, bosses should be beefy on every account because they're boss monsters and because they're a "blood-chilling threat". It's kind of not bossy nor blood-chilling when something dies from 1-2 hits without even having the chance to retaliate (lv 14-22 mist elf duelist, about 40 dmg before PV on average). I've met five or so bosses, and they put up less resistance (read: none) than ogre lords, ratling duelists, annis hags, stone golems, dwarven chaos knights, and hill giant chieftains I met in the same levels. And those aren't exactly endgame stuff either.