It is certainly possible, but for the sequence to work, it’s going to depend heavily on several factors.
1) What the style that they are learning is.
The meaning behind “Slightly better trained” or having spent slightly longer training can change drastically based on which the style the characters are learning.
Not all martial arts and combat forms are created equal, each one is designed for a specific purpose, and like any form of technological advancement, it changes as it evolves. The police and the military both use eight week training courses in hand to hand that are designed so that the soldiers and the cops can pick the techniques up very quickly. These styles have a hard limit, because the students don’t have a lot of time to adjust to them. They learn what they need to know and move on. Soldiers who come out of these training programs end up fairly equal, for the most part but the difference between two trainees who are separated by a week or even just a day of training is drastic.
Some styles that are worth looking into: M.A.P. and CQB (US Military), CQC (US Police), Krav Maga (Israeli Defense Force), Systema and Sambo (Russian Special Forces).
Compare these to a styles to a combat art like Shotokan Karate or Taekwondo, where it can take years (if ever) to achieve any kind of real combat proficiency. Depending on the style, the belt ranking system, the school, and the instructor, two students with a single belt rank of difference like a brown belt versus a blue belt or a yellow belt versus a green belt can be fairly even. However, these styles are designed to be picked up at a much slower pace than military forces can allot time for. Now, many martial arts practitioners do end up stronger combatants in the long run (if they’ve been learning how to fight in the real world), but it takes a substantial time investment that works better for an individual than in a system where high turnover is necessary.
In short: it could happen, but that scenario is more likely to occur in a martial arts dojo than between two soldiers of different levels of training. It’s also fairly likely to happen between two soldiers who’ve both completed their training course and one has spent slightly more time in the field. Now, it can happen over the course of training, but you’re ultimately going to get more mileage out of characters who’ve come up together at the same time with one simply having been singled out as the “best” in the class. This way, instead of having to surmount an accelerated training course, the characters have to get over their own mental obstacles. Which brings us to number 2.
2) The mind plays a key role in victory or defeat
It’s important to remember that people aren’t machines or like characters in a video game. You can’t simply slot X allotment of powers onto them and expect them to perform at maximum capacity all the time. A belt ranking system in a martial arts dojo isn’t so much a sign post for how good a character is, but what they’ve learned, overcome, and accomplished in the course of their training. Depending on the school, a brown belt can defeat a lazy black belt if that black belt doesn’t continually put the work in to keep their skills sharp. If someone hits the top and just rests on their laurels, they’ll get taken by an up and comer assuming that we as the audience (in a story context) have seen the character putting the extra work in to get to that point. The other half is that the brown belt is going to have to work very, very hard to defeat the lazy black belt, simply on the grounds that the black belt knows so much more than they do.
A character can be off their game for any number of reasons and, ironically, most of those reasons don’t boil down to overconfidence. It could be bad news from home, the wife serving them with divorce papers an hour before the fight, strain from surviving their first battle out in the field, or what they witness during a time of war. It could even be something as simple as what they are and aren’t allowed to get away with in a training context. What someone is willing to do on the training floor against one of their buddies can be different from what they’ll do to a complete stranger on the battlefield.
The character with less training winning the duel is ultimately going to rest on how the other character is feeling that day, and on one other crucial bit:
3) Can the character take advantage of someone else’s mental state?
You never want to write a gimme, especially with a fight sequence. The duel between these two characters could set the stage for the rest of the book and regardless of who comes out on top, it should feel that the character earned the victory or, if done well, they don’t but it’s not from something they did. A character can win a fight but leave the audience unsatisfied, if that’s intentional and leaves the POV character unsatisfied as well then great, but if it’s unintentional then it’s a problem.
You’re going to walk a very thin line here because a better character being cocky and letting their guard down is cliche. Now, I’m not saying don’t do it. Cliche works for a reason, but you have to remember that it is and work extra hard to make sure it doesn’t feel like every other version of this fight I could find by just picking up a book off my shelf.
Ultimately, Character B wins X because Character C is a cocky asshole has been played to death. The Character B also feels like they’ve won on a technicality and not through anything they did. If they do win on a technicality, then that doesn’t really tell the audience anything or give them a reason to invest in the character. However, if Character B is shown working very hard to defeat cock asshole Character C and Character C loses because they assumed X about Character B, when B moved forward off that point, then the victory will be satisfactory to the audience. A character who works hard to win, who works hard to be more than they are, and who earns their victories is always more appealing in the long run than a character who is “just that good”.
You’ve got to balance them both, but an outside reason for victory is never as good as an internal one.