That’s… really vague. The part about what she needs to know. Knowing nothing about your world, I can’t actually answer the question of what she does or doesn’t need to know how to do. Think about the threats she faces on a regular basis. Think about her support network. What resources does she have access to to defend herself? Who is/are the enemy? If there are multiple ones, which does she face the greatest threat from on a regular basis? Is she in a privileged position in her culture or a gutter rat? Does she have a solid connection with friends and family or does she have to go it alone most of the time? What threats are there in her environment? Gangs? Organized Crime? Corrupt Police? Invading military? Local Military?
For all I know, the major enemy she has to be on the lookout for are man-sized, bipedal rats that crawl up out of the underdark.
Someone had to teach her how to fight, if she’s any good at it. The question is: what did they teach her? The answer is: whatever is the most basic and common skill set readily available that is necessary to ensure her survival. If so, somewhere in there, this someone probably taught her the virtue of running and hiding which is a skill all children who live in dangerous societies must learn because fighting adults head on is a losing proposition. This brings us to the most inglorious and useful skill when fighting groups: the art of running away.
The problem with groups is that working together is the natural human state and, even with no actual combat skill, they can be very dangerous just working off of base instinct
Have you ever just stood back and watched a game of keep away? It’s a really cruel thing young bullies like to do, some of you out there have probably been on the receiving end. That is, at it’s most basic, the basic strategy employed by groups in combat. Distract the target up front, then keep them off balance and having to chase the object as it moves from person to person. When you add violence into the picture, one individual distracts while the others flank outside the range of peripheral vision. This may be by talking, it may be by attacking, but the goal will be to pen in the target and attack the areas where they are most vulnerable (the back, the kidneys, the spine). Once the person falls (which happens very quickly), they continue the assault by stomping them and kicking them until they pass out/die.
The real world/self-defense advice for dealing with groups is: don’t. Run. If you’re forced into a fight, cripple your opponents so you can extract yourself and then run. If you have to fight them and know the terrain, run so they get strung out and you can take them individually. Remember the eight move limit until physical exhaustion. The individual can only throw eight moves total without, at least, a small amount of rest. Each member of the group can make eight moves and by switching between fighters (pressing in two on one), the other two (or three) can take moments to rest which allows the entire group to fight longer.
In these articles, I go into it in more depth.
Hopefully, these will be enough to get you started.