Perhaps in the fanfiction, there's a misunderstanding. Guy Sensei thinks Tenten has stolen a valuable jutsu scroll or something important to him. Naruto, knowing Tenten is innocent, stands in front of her. He might try to reason with Guy Sensei first, using his charm and quick - thinking. If that doesn't work, he'll be ready to fight, using his Rasengan if necessary.
Naruto could use his shadow clone jutsu to create distractions. While Gai is busy dealing with the clones, Naruto can quickly get Tenten to a safe place.
In the context of fanfiction, Gi Sensei might be after Tenten because she has some unique knowledge about an ancient ninja art. He believes that by capturing her, he can extract that knowledge for his own gain. Or maybe he was hired by someone who has a vendetta against Tenten. There could also be a misunderstanding where Gi Sensei thinks Tenten has wronged him in some way, even though she hasn't, and that's why he's targeting her.
His relationships would also develop. By protecting Tenten, he would earn her trust and respect. This could lead to a stronger bond between them and also with other members of their ninja group. Naruto might also gain a new perspective on Guy Sensei, understanding that even strong and respected teachers can be vulnerable to certain influences. He would learn to look beyond the surface and be more empathetic towards others.
Well, in fanfictions there could be various ways. He might use his Rasengan to create an opening in Gi Sensei's defenses and then rush in to save Tenten. Another possibility is that Naruto uses his speed and agility, enhanced by the Nine - Tails' chakra perhaps, to dodge Gi Sensei's attacks and reach Tenten before she gets hurt.
It could be that Might Guy is under some sort of mind - control. So, without his normal self - control, he attacks Tenten, making it necessary for Naruto to save her.
Naruto could use his Uzumaki bloodline's chakra reserves. He might overwhelm Might Guy with a large number of shadow clones, confusing him and creating an opening to save Tenten.
Well, perhaps in this fanfiction, Might Guy is possessed by some sort of dark force. This makes him turn on Tenten during a sparring session. Naruto, being the hero he is, arrives just in time. He first tries to talk to Guy, but when that fails, he uses his speed and agility, skills he's honed over years of fighting. He quickly grabs Tenten and gets her to safety, then he has to figure out how to free Guy from the dark influence.