When NOT to use the hero's journey mentor pattern

The Mentor character is a very familiar one, especially in hero journeys and coming-of-age stories.  The pretty classic formula for the mentor (Obi-Wan, Gandalf, Dumbledore) goes like this:

After the protagonist becomes committed to the quest, the mentor provides advice, help, and tools.  Once the quest gets tough, however, the mentor disappears (or dies) and doesn’t come back for a long time.  If the mentor does finally come back, it is in a different capacity: he can sometimes advise or deal with events elsewhere (or maybe he just left a helpful diary), but he can no longer personally lead the hero through what only the hero can do.

This formula worked just fine in Star Wars, The Lord of the Rings, and Harry Potter, because those three stories following the coming-of-age of the new guard (SW and HP) or the lone adventure of an everyman (TLOTR).  But what happens when the protagonist is in a power growth fantasy?  Take the following story:

Y is a high school student who is possessed by a dangerous magical entity but who manages to retain control.  He slowly begins to be able to use the entity’s magic himself, but in the process, the entity becomes more and more powerful.  If he doesn’t keep the entity under control, the entity will destroy everything.

Enter Professor G.  Professor G is the strongest sorcerer in the world, although only in his mid-twenties.  In order to fight internal corruption in the magic world, Professor G has been recruiting and training sorcerers who have the potential to be as powerful as he is.  Impressed at Y’s potential, Professor G brings Y to his magic school and begins training him.

So far, we have a pretty classic beginning, but there are two big differences here between this story and Star Wars or Harry Potter, and those differences are important: first, Professor G is less than a decade older than Y, which means Y can’t be the new guard replacing the old guard—they’re the same generation.  Second, Y is explicitly said to have the potential to be as powerful as Professor G within the course of a few years—which is certainly not the case for Harry or Frodo.  (Luke has the potential to be more powerful than Obi-Wan, but, as with the others, the age gap is huge.)

There’s one more element that I haven’t mentioned, but which is very important for story and character development: Y is a fairly classic, good-hearted and grimly determined hero, whereas Professor G is silly and extravagant.  This not only creates a very different interpersonal dynamic than the classic hero-mentor dynamic, it also lends the story much-needed variety.

Now, given this information, let’s plug in the classic next step for the mentor (which is what the story does):

When things become dangerous, the villains specifically target Professor G and trick him into being trapped.  Now Y is fighting them alone, without the help of the world’s most powerful sorcerer, and the situation is dire.

And here is where everything falls apart.  Let me explain:

1. The Protagonist Looks Pathetic #1: The author (irl) got Professor G out of the way because G was overshadowing Y by being way cooler and more fun than him.  But how pathetic is that?  You don’t make you’re protagonist more cool by getting rid of everyone cooler than him; if he isn’t cool enough on his own, that’s a him problem.  Besides, man is judged by the company he keeps.  Y is cooler because Professor G respects him and interacts with him.  What’s more, Professor G provided an excellent foil, and without him, the story becomes a one-note grind.  (Also, you got rid of the best character, you dunce.  I’m not going to keep watching your show now, though I will look up how it ends.)
     Solution: Make Y cooler through keeping his association with Professor G

2. The Protagonist Looks Pathetic #2: The entire premise of the show is that Y has the potential to be as powerful as Professor G.  So if you have to get rid of Professor G to give Y a challenge (since otherwise Professor G would easily solve everything), what does that tell us about Y’s power level?  Besides, since Professor G was the scale by which power is measured in this world, any power growth apart from him is meaningless.
     Solution: Make Y look more powerful by showing he has become worthy to fight alongside Professor G

3. The Villains Look Pathetic: If they can only be a threat when Professor G isn’t around, they aren’t a threat.
     Solution: Make the villains so powerful that they’re a threat even when Professor G is around.

4. Characterization Is Ruined: Professor G is depicted as someone who can only be delayed but never truly stopped, and who endlessly does the impossible.  So the fact that he doesn’t ever figure a way out of the trap by himself . . . is unbelievable and annoying.
     Solution: Keep Professor G in character by having him find his way out. (Bonus: this may itself cause further interesting complications)

5. It Isn’t Awesome.  This is an extreme power-growth magic story.  Having the protagonist not look pathetic isn’t enough; we need the most awesome fight and magic visuals which are full of personality for a variety of battles.  Getting rid of some of the best visuals, and visual variety, is foolish.
     Solution: Include the most awesome climactic sequence by utilizing both Y and Professor G’s powers, personalities, and interactions to the max.

If your goal is to make your protagonist look strong and cool and awesome in a power-growth fantasy, show that he can fight as an equal next to the world’s strongest sorcerer . . . not that the only way to make him look good is by getting rid of everyone better than him.

Note to anyone who figured out which story I'm talking about: yes, I do know how the rest of the series goes and how it ends, and I'm sorry to say it only doubles down on the error I outline here.