Thought I’d chime in here since I’ve responded to these threads before and it’s more or less my area of expertise (I’m a contributing editor at Black Belt Magazine and the author of The Principles of Unarmed Combat). Without knowing more specifics about the scenario you have in mind (rules on the use of weapons or illegal techniques, how winners are declared, etc.) it’s difficult to offer too many suggestions. But to address a few points that other people have made:
If you’re looking for any level of realism, a normal sized woman is not going to get enough from one month of training to defeat the average man in a fight short of catching him by surprise, such as kicking him in the groin when he’s not expecting it. Of course, if she has to fight another opponent after this, and he’s paying attention, such ploys will become increasingly difficult to use effectively. The style of fighting she learns won’t matter very much as one month is simply not enough time to acquire any significant skill and the average woman is going to be smaller and weaker than the average man. This is assuming you are making her of average size with no prior physical training and her opponents are not complete wimps or fools.
That being said, it is fiction and you can always make it as unrealistic as you choose. While there is no one “best” martial art, if you insist on having her train in a martial art for a brief time to give her an advantage, the most effective one would depend on the nature of the rules you would be employing for the fights and what the skills of the opponents are. However, certain martial arts, despite what others have claimed, would not generally be very effective. Aikido, while a beautiful martial art to practice, is not typically regarded by serious close quarter combat experts as a practical martial art for fighting. Classical Japanese ju-jutsu (not Brazilian jiu-jitsu) was largely designed to be performed as a supplement to armed combat, often employing the use of a knife or other weapons and only being used empty handed as a last resort when you had lost all your own weapons.
Perhaps the most (semi) realistic way to make such a scenario partially believable is to mirror what was done in the early Ultimate Fighting Championships tournaments. These events were largely dominated by grapplers (Brazilian jiu-jitsu practitioners as well as shootwrestlers and sambo practitioners) who won matches by simply dragging their opponents - often larger, stronger men who were skilled in stand up fighting but had little experience in ground fighting - to the floor and turning the matches into pure ground fighting battles. They were able to more effectively employ leverage and use submission holds such as chokes or straight armlocks, which required less pure size and strength to perform effectively than many standing techniques do. This, of course, would mean your main character would train intensively in the basics of one of these grappling styles while her male opponents would have no real knowledge of or skill in groundfighting. It would also probably mean rules which precluded the use of biting or eye gouging, which tend to make employing groundfighting maneuvers far more difficult (though there is at least one style I know of called Hkyen, which comes from Myanmar/Burma, that does make extensive use of biting techniques to go along with its grappling).
Hope some of that helped.
