She has little interest in them, as said before the Empire didn't consider boxing to be a vital form of survival when fighting the Nosferatu, they believed more in a combination of martial arts that would train the solider to be faster to make up for their lacking the direct physical strength that the Nosferatu had. For them learning how to run on walls as well as being able to keep on the move and use their speed to their advantage in order to keep alive was more important than being to stand toe to toe with an opponent that could rip them apart with ease.
Against someone who is trained at boxing, Reina wouldn't have a chance against sheer strength, but she would have the upper hand on keeping her momentum going and wearing her opponent down until she can attack.
Reina was taught to defend, kill and survive form the age of three, she still trains even though she has no Empire to serve anymore. The mantra for Empire soldiers was very simple: Keep moving or die. Against a boxer she would try to keep away from the attacks by getting her momentum going, once her momentum gets going she can't be stopped easily, she's fast and agile - as required for Empire soldiers. She wold duck, slide, back away quickly, block and keep moving around and not give her opponent time to trap her. If trapped she'll fight as a trapped animal would - she would lash out in an attempt to stun and stagger her opponent, then use that chance to either kill them or disable them.
She would try to hold back on killing though unless she has no other option.
Neggy's got better physical strength though, as required for a boxer, it may not be at the same level as a male would be, but even a female boxer has to be physically strong. That gives her an advantage over Reina, Reina's trained to be fast, she has average physical strength but not at the level of a trained boxer.
Her advantage would be her speed, if she can keep on the move and try to wear Neggy out then she could have a chance, but only if she can avoid any serious punches. Her healing rate has it's limits, the more the damage and the more she heals, the faster she becomes too exhausted to keep going and will either drop into a coma to save her higher brain functions or her medical requirement will kick in to do the same thing, leaving a pretty helpless toddler in a pile of clothing.
It would be an interesting fight to see, both have their strengths and weaknesses and both have an upper hand over the other, but only if they can make use of that upper hand at every chance.