Everyone loves a necropost!
But, for anyone googling like myself that ends up here, I have a bit to add. The rule book already has some information that can be used to make a decent NPC with level into monster with CR. The hidden nugget is in the druid section, paraphrased as "when using wild shape you keep your mental data, you switch out your physical data to match the animal you change into". That allows you to play a monster (beast) as a player character, and I suggest the same applies in the other direction.
You can create a NPC with normal stats, abilities, spells, and feats, but when it comes to the physical characteristics model after other monsters who's CR is roughly 4x -5x the NPC level. These could be the Nonplayer Characters section of appendix B in the monster manual (p.342) or for higher level CRs could model after celestial or demonic monsters. This allows you to build and maintain your antagonists personality via the mental stats, but allow for longer fights with positive feedback from more successful attacks due to the physical stats.
It's certainly not a perfect solution, but it's more inline with existing game mechanics and will make a more robust enemy for the players to battle. I didn't see another post suggesting an idea along the same lines, so I wanted to at least through it out there for people to contemplate.