She and Myrtle had learned this in their previous life, but Myrtle was stronger than her in the physical aspect. This skill she developed was completely lost after she married Alistair, and her stamina dwindled over the years.
If Adrienne's parents knew what she was doing, they would oppose it. However, since her father didn't have an ounce of care for her and her mother was deep in a coma, Adrienne could only rely on herself for protection. She knew the basics, but her body needed strength and stamina to utilize it.
Amateurs were thought to focus on basic boxing skills as most people find it relatively intuitive to strike using their hands. Beginners will also learn how to jab, cross, hook, and basic front and side covers for defense. This was enough for Adrienne for now. She didn't need to use complicated skills to subdue these arrogant girls.