Dude, you'll have to crack open the Dungeon Master's Guide and go to the encounter building chapter to discover what's happening under the hood with Larian's BG3.

Essentially BG3 presents you a level which is an amalgamation of character level, challenge rating and encounter level. Turns out to control the danger level of fights a referee needs 3 such values. (Or you can eyeball it and see what happens.)

Anyway, Kagha, is built like a monster and has little-to-no fiddly features of a character. It is instead aggregated into a single HP pool. You noticed so many people-monsters have so many HPs? They're not built with class and levels; they're built as a challenge rating.

Your incredulousness is understandable. Tactician is inflating people-monsters to boggling amounts. Guards don't normally have 90+hps, those are named notables in the city.

Now if you do build people-monsters with character classes you'll notice immediately how few HPs they have and how many fiddly powers they possess, and that fiddly powers mean nothing because they're dead round 3. Basically, combat is rocket tag.

If you get all narrative with ability scores, as some DMs do, the HPs can be really pathetic. An elderly cleric 10 might only have HP35. That's fine for TT. CRPGs? Not so much.

And this all gets to the crux of why I don't want a higher difficulty with more inflated numbers; it starts attacking the narrative of the game world. HP420 Grym? Sure, it's a legendary guardian made from adamantine. HP300 Surgeon? Um no. Should be in the 150 range, or at least highlight better his 1000 year lifetime rather than tuck it away in a diary.