So in +Ben Milton's review of The Black Hack he voices the concern that TBH's Powerful Opponents mechanic doesn't account for things like tough monsters which are slow but easy to hit, or monsters with few hit points that are hard to hit, and other such edge cases; how hard a thing is to hit scales linearly with its hit die: the system lacks nuance.
As a cursory rebuttal, I'm a fan of the system as is. I think of it as not an actual physical representation of how hard a thing is to hit but a mechanical abstraction of what +Pearce Shea is talking about when he talks about monsters: just being around them makes your life worse. That something so much more than you is trying to kill you just makes everything you do demonstrably harder.
However, if what Ben raises is an issue for you, never fear, I am here to dispense ludomantic wisdom. Or rather, just take it from somewhere else and tweak it slightly.
Anyway, it's pretty simple:
For certain monsters, don't roll hit die. Just assign health.
This idea is taken from how S&W Whitebox deals with dragon: you don't roll their hit die, you just assign hp per hit die based on their age (young dragons have 1 hp/hd, ancient have 8, etc).
So for tough but easy-to-hit monsters give them 8 hp/hd or something, but lower their total amount of HD, and vice-versa.
So ogres might be 2 or 3 HD creatures: overwhelming for a rookie adventurer, but not so for one with a few adventures under their belt. But give it 8hp/hd and its still a big beefy threat.
Similarly a wyrmling might still have 9 HD, but only 1 or 2 HP/HD. It may be young, but it's still a dragon: it'll always be terrifying and primordial. To bring up Pearce again, monsters may not be all that terrible mechanically, but it's the things around them that affect you. It really gives the PCs an oh-shit moment when I tell them to roll at a -8 or 6 or whatever and they do the math and they realise this thing has nine fucking hit die how will we kill it holy shit, etc.
Plus it allows for one lucky strike to kill one, giving you that really folkloric dragon slayer feel like St. George or Wiglaf.
EDIT: I had a game involving Players vs Monsters vs Monsters (or rather, a group of NPC adventurers vs Monsters. My casual work-around is to make the Monster "stat" 10+HD. If you feel like the monster is particularly good or bad at a particular stat, make it 10+HD+/-(1-3/d4-1). This can make things wonky if you're using this little HD as AC conversion thing I've got going on, but I've found a work-around that still keeps the stat-block to 3 pretty easy numbers. There's the HP/HD value, the number of HD, and the HD it damages/tests as. If you make a low-level monster have "high AC" by giving them higher HD, don't use the regular TBH HD/damage scaling. Just give them appropriate damage. And vice versa for strong monsters with "low AC".
Let's put this into practice with the notation system I use to show a few example stat blocks.
OGRE: 6/4/3HD, TN 13(+2 STR/CON, -2 DEX). Can INSULT for D6 damage, CHA save. Can eat anything given enough time and ketchup.
To explain what all those numbers mean, the first number before the slash is how many HP/HD to assign, the second number is the HD it deals damage as (i.e. Ogres deal damage as 4HD creatures, doing D12), and the third number is how many HD it actually has. The TN is what it needs to roll below if it ever comes up, plus relevant stat modifiers. As you can see, the TN is calculated using its "test" HD value. Everything else is it's special abilities, so really the actual stat block is still quite small.
So that's a tough but easy to hit monster. The other end of the spectrum is:
ARMOURED GUARD: 1/2/6HD, TN 12. Can use Spear 1H or 2H.
So this guy is hard to hit because he has high AC, but he's squishy underneath that armour and not that capable if he ever has to test himself. He also gets the relevant Armour Points from his HD (5) which adequately represents his AC along two vectors: damage avoidance and damage reduction. Ever seen another RPG represent both aspects of armour that neatly?