Perhaps Dumbledore's long - lost brother could be the trainer. He might have been in hiding all these years, mastering some extremely powerful magic. He decides to come out of the shadows to train Harry in order to prepare him for a great battle that is to come. He could teach Harry some lost arts of magic that were once known to the founders of Hogwarts.
It could be Sirius Black. Sirius was a very powerful wizard and he had a special bond with Harry. He could have come back in some way to train Harry in some unique magical skills, like how to transform into a more powerful animagus form.
Another possibility is that it could be a powerful centaur. Centaurs are known for their wisdom and magical knowledge. One of them could take Harry under their wing and train him in the ways of nature - based magic, which is different from the magic taught at Hogwarts. Harry would learn to communicate with magical creatures in a deeper way and use the power of the natural elements in his magic.
Well, perhaps in this fanfic, Harry Potter could be receiving special training in advanced defensive magic from a reclusive and powerful wizard. This training might include learning spells that are not taught at Hogwarts, like some ancient protection charms.
It could be Hagrid. He has a lot of strength himself and might know some basic principles of building muscle. He could guide Harry on how to lift the heavy weights, using his own brute strength as an example.
There's a chance that it could be an old and powerful wizard from a far - off land. This wizard could have unique magical abilities and training methods that are not known in the British wizarding world. Harry would be exposed to a whole new set of magic and training styles, which would make the fanfiction more interesting.
It could be a group of fans who collaborated to write this fanfiction. They might have different ideas about what 'receives a tri' means and combined their creativity to come up with a story. Maybe they met in an online Harry Potter fan community and decided to work together on this project.
It could be an amateur writer who is a big fan of military - themed stories and the character of Harry. They might have created this fanfic to explore the idea of heroism and recognition within a military context.
There's a chance it could be a powerful house - elf. House - elves are known for their own brand of magic, and one particularly rebellious or enlightened house - elf could decide to train Harry in magic that is more about stealth and quick - thinking. After all, house - elves have to be very sneaky to avoid detection by their masters most of the time, and this kind of skill could be very useful for Harry in his battles against Voldemort.
It could be Dumbledore. Even if he's dead in the original story, in fanfiction, anything is possible. He might have left behind some kind of magic that allows him to train Harry from beyond the grave, or he could have faked his death to take Harry to a secret location for training.
Maybe it's a former Death Eater who has seen the error of his ways. He wants to atone for his past sins by training Harry to defeat Voldemort. He has inside knowledge of the dark side's tactics and uses this to make Harry a more formidable opponent.
I think it could be Neville Longbottom. Neville has grown into a strong and confident wizard. He could have discovered boxing as a form of self - defense and then decided to teach Harry. Neville would bring his own unique perspective, having been bullied in the past. He'd train Harry to be both strong and fair in the boxing ring, emphasizing the importance of honor and respect in combat.
It could be Dumbledore. He's always been a guiding figure in Harry's life, so in the fanfic, he might continue to mentor Harry through his special training, teaching him ancient and powerful magic.