I was just looking at the monster listings in the D&D Basic set once more, and figured that the average Dark Dungeon combat skill should be higher for lower level monsters. Otherwise they mostly wouldn't make much sense, with warrior races like goblins and kobolds having no actual combat skill. And some monsters like werebears should be extremely fierce. So, here's an alternate table for hit dice to attack skill.
D&D HD DD Combat Skill Example
untrained human 0 peasant
less than 1 2 kobold, halfling, goblin
1 3 hobgoblin, berserker, elf
2 4 lizard man, ghoul, zombie, wolf
3 5 wererat, wight, bugbear
4 6 werewolf, medusa, panther
5 7 weretiger, lion
6+ 8 werebear, minotaur
Above 6 hit dice, you should really look at where the dice come from. Is it size, or is it high agility, or is it even more combat skill? Dragons would be an example of great size. So I would scale down their combat skill by two or three points, but their armor up by one or two. And I'd advice varying levels for individuals anyway.
So a white dragon of six hit dice would have armor 3, but combat skill 5.
And a red dragon of ten hit dice would have armor 5, but combat skill 8.
Or something along these lines.
Skill 8 is pretty fierce, with a 96% hit rate (roll 9+ on d10 + 8 and don't fumble on a 1), 70% special hits (roll 12+ on d10 + 8), and 40% critical hits (roll 15+ on d10 + 8). I would prefer not to take these on if I could help it! And armour 5 is impossible to seriously take on with all normal weapons, so a Red Dragon is a true M-O-N-S-T-E-R this way. Cool.
Never thought I'd say that.