Min/Max Boards

The real question is whether or not monsters can have class levels by RAW. I know they can if the DM is building a custom monster, but I don't think it is called out in the MM.

Yep. I fired the question over to the sage advice thing. Maybe we'll get an answer. Maybe not. Who knows?

Still, so far as I can tell, there's nothing to say they can't. And the idea that a DM can give one levels indicates that they're capable.

Because otherwise True Polymorph is actually worse than killing them. Kill them, and they can be raised. True Polymorph them into a frog or something and they're fucked. You can't even use it to turn them BACK because it would lock their CR at 0, meaning the only other things they could be turned into would be equally weak.

True Polymorph can let a player continue advancing beyond 20th level. Depending on DM of course.

Party hit's 20th level. Everyone starts shaking hands and going "Yeah, well maybe I'll roll up a bard next time." And then the wizard goes "Wait, I have a plan. We don't have to end here!"

And then the plan goes something like this.

True polymorph replaces your game statistics entirely with the transformed form with the exception of alignment and personality. If concentrated on for the full duration, the target form becomes your base form. So your old character sheet becomes worthless. You turn your party into a CR 20 monster of their choice. Ancient Brass Dragons are nice for everything except Rangers, Rogues, and Monks, for reasons to be explained later.

Ok. Everyone is monsters forever. Now what? Well. You start over as a first level character. A first level character who happens to be a CR 20 monster. Since XP doesn't scale, you'll breeze through the first levels. But wait, monsters can't take class levels can they? Well. There's no real reason why not. According to the Multiclassing rules, in order to gain levels in a class you either need natural aptitude (A 13 or higher in the chosen classes required ability score) or years of training that was presumed to have happened before first level.

Since you chose a monster form conducive to your new chosen class, And first level requires 0 XP, which you have. You then take first level in whatever class you were before. An Ancient Brass Dragon has high enough scores to pass muster in everything but Dexterity, which is why they can't be Rangers, Rogues, or Monks. But I'm sure there's another critter around for those who need dexterity.

And then you just play from there. When you hit 20th level again? Do it again. Switching over to a BETTER dragon.

For wizards this trick is especially good if you set aside your spellbook prior to transformation. As you keep your original spells in your spellbook, but you also get another two every level. Meaning after enough cycles you'll have all the spells you could ever want.