Throughout the books we see Molly as a kind, selfless and caring woman who goes out of her way to help others and make them comfortable despite whatever she and her family may be going though. We also know that she had a tragic past as she lost her brothers Fabian and Gideon Prewett in the first wizarding war. Through the course of the first 6 books none of the readers could have guessed that Molly was not only an amazing mother and host, but also a fabulous duelist. It's likely that after losing Fabian and Gideon she was determined to toughen herself and participated in duels with Arthur to improve her skills. Not many witches could have taken on Bellatrix Lestrange alone. We saw during Deathly Hallows that Bellatrix was able to duel three people at once without breaking a sweat. Molly was able to hold her own while duelling Bellatrix.