Well, 'Combat - Focused Harry Training' is also worth checking out. This fanfic focuses on combat training but also adds a bit of a twist. Harry not only learns physical combat skills but also how to use his mind in battle. He has to overcome his fear and use his strategic thinking, along with his newfound combat skills, to face the challenges in the story.
One popular 'harry training fanfic' could be 'Harry Potter and the Training of Champions'. It focuses on Harry getting special training for various magical skills. Another might be 'Training with the Order', which delves into Harry's training under the Order of the Phoenix members.
There's no way to know for sure without reading the fanfic. It could be some well - known Death Eaters like Bellatrix Lestrange, Lucius Malfoy, and Severus Snape (although Snape has a more complex relationship with Harry in the original story).
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.
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.
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.
Harry would become more disciplined. His natural leadership skills would be enhanced by the military training, making him even more of a force to be reckoned with.
One interesting plot could be the main character facing a series of extremely difficult and dangerous dark magic challenges during the auror training. Maybe they have to deal with a powerful dark wizard who has infiltrated the training grounds to disrupt the training process.
In some fanfics, Harry might go through combat training with new spells and dueling techniques. This could include advanced defensive magic like creating more complex Patronuses.