Two Sets of Rules, the Good and Bad?

[MaxWilson]

Should be able to. Absolutely. Should not be considered "doing it wrong" for doing so.

However, doing it that way is extra, unnecessary work every time I've tried to do it (which I've tried). And doing so has led to less-successful monsters than doing it the "normal" way.

...

And he, not she or they. But I don't particularly care.

Sorry, "he." Didn't want to assume without a picture.

Anyway, if NPCs with class levels are difficult for you, it's fine for you to avoid it. No one is telling you that you have to use them. But sometimes just plopping a 7th level Diviner with 46 HP into an adventure is simpler than adjusting an MM Mage template to be 7th level and recalculating the CR, which is a nightmare if the spell loadout includes spells like Hypnotic Pattern, Lightning Bolt, Polymorph, Evard's Black Tentacles, and Tasha's Hideous Laughter.