I'm running a D&D sandbox (with my own variant of D&D, of course, but it is reasonably compatible with old versions of D&D and various retroclones), and I use several adventure modules and random tables that are made for various rule systems (not all of them level-based), and sometimes for no system at all. In this situation I often have to create game statistics for various creatures and non-player characters.
We play so that the players are responsible for their success. The referee uses pre-created material, random tables, established rules, and (when necessary) improvisation to run the game. The referee is (as much as possible) a neutral referee, and does not for example choose how difficult encounters are, what is encountered, or how hostile they are.
For this reason it is important to be able to establish game stats for a creature without thinking about how challenging the encounter is or should be. This is easy for everything but monster hit dice or levels, which are fairly disassociated from the fiction.
How can I assign hit dice / level to a creature based on its fictional description only?
Something like Dungeon World monster creation or Burning wheel monster burning is okay, since they rely on the nature of the creature within the fictional world. Pathfinder monster creation is completely unsuitable, since it starts very strongly by deciding the CR of the creature, and CR is not a fictional detail of the creature.
I am looking for both published methods and methods people use in their house games.
A good method would be one that can be used on-the-fly during play, with all the time constraints that implies. This is not necessary, but would be nice.
Two out of three adventurers died when the third one abandoned them (and wizard locked them in with a murderous undead thing). They'll rise to plague the traitor as corporeal spirits of vengeance if someone is foolish enough to open the wizard locked door. What level should the vengeful dead be? (Both characters were first level when they died.)
Failed spellcasting resulted in a character summoning an angel, bright as anything, with a burning sword and four wings. I also have game mechanical stats for the seraphim, but they are for Burning Wheel (in the Monster burner). What level should it be?