Just thinking again about how to improve 'boss monsters' like the Witch Lord in the classic EU ruleset so they don't die too quickly. (I might have already posted a thread on this years back but can't find it now.)
There are two issues to my mind: Body points and Spells.
As usual, my approach is to try to keep things as simple as possible, and then make it even simpler!
Body points
Infamously all monsters in the EU base game, Kellar's Keep and Return of the Witch Lord had only one Body point each, apart from that special Gargoyle at the end of KK. When people hear that the North American edition gave monsters extra BP (as does the Hasbroquest remake), they usually think, "Cool! Much better!" And I used to think so too. Back in the day I upgraded my Fimirs, Mummies and Chaos Warriors to 2 BP each, and the Gargoyles to 3 BP.
However, over the years I've become more and more dissatisfied with giving ordinary monsters extra BP. In practice it just seems to slow the game down to a slog, since it can take a few turns just to get one skull past their multiple defence dice and HQ combat isn't that interesting. It also requires some slightly annoying bookkeeping (such as turning the model around to show that it's been hurt, or putting a die beside it, etc - counters in the NA edition). My younger self would be aghast to hear it, but I do think the UK designers got that part right, even though it never felt right, if you see what I mean.
But boss monsters are another story. It's pretty embarrassing for the Witch Lord or Ograk the Orc Captain to die in a single hit before he can even fight back. The UK designers clearly agreed, since they started giving Chaos Sorcerers and the like individual 'body point tracks' to check off in Against the Ogre Horde and Wizards of Morcar.
The question is, then, how to determine the Body points of boss monsters in the base game, KK and RotWL. What method do you use to decide this?
My own thinking at the moment, keeping it as simple as possible (and keeping the amount of BP as low as possible), is as follows.
Is it a personality monster?
If so, look at how many Defence dice it rolls. Halve that number, rounding down. That's how many Body points the monster has. Keep track of the monster's Body points on a spare character sheet.
(The term "personality monster" comes from the Against the Ogre Horde EU questbook, and refers to a unique monster with a special characteristics block and usually a name. It's very British and I like it.

Looking through the base game, KK and RotWL, the monsters that qualify for more than one BP are:
Base game
Ulag - Body 2
Grak - Body 2
Balur the Fire Mage - Body 2
Witch Lord (base game) - Body 3
Kellar's Keep
Ograk - Body 3
Special Gargoyle - already has 3 BP
Return of the Witch Lord
Bellthor - Body 3
Skulmar - Body 3
Kessandria - Body 3
Witch Lord - Body 3
By first impressions at least, this feels about right to me, by contrast to the ordinary monsters with only 1 BP each.
There are a couple of other 'leaders' such as Verag in The Trial, but they don't have special stat blocks so they just fight as ordinary monsters with 1 BP. There are also special monsters like the Dwarven-king skeletons in KK or the Spirit Riders in RotWL, but they're not unique monsters so they also just have 1BP each as their stat blocks say.
Kellar's Keep is admittedly light on multi-BP bosses under this system. The one I have some qualms about is Petrokk, a random Chaos Sorcerer in 'The East Gate', who is likely to get demolished the moment the heroes meet him because he's all by himself in a room close to a door and probably won't have time to get a hit in. But he's just a side character and not the 'boss' of that quest (arguably that's the dreaded shapeshifting Fimir) so I can live with it.