I know the question of which spell was used has been asked (without really an answer) before. This is not a question of which spell or the series of spells cast. My question is how did Molly have the power to kill Bellatrix.
Harry Potter Book 7: Deathly Hallows:
Bellatrix was still fighting too, fifty yards away from Voldemort, and like her master she dueled three at once: Hermione, Ginny, and Luna, all battling their hardest, but Bellatrix was equal to them, and Harry’s attention was diverted as a Killing Curse shot so close to Ginny that she missed death by an inch—
He changed course, running at Bellatrix rather than Voldemort, but before he had gone a few steps he was knocked sideways.
“NOT MY DAUGHTER, YOU BITCH!”
Mrs. Weasley threw off her cloak as she ran, freeing her arms. Bellatrix spun on the spot, roaring with laughter at the sight of her new challenger.
“OUT OF MY WAY!” shouted Mrs. Weasley to the three girls, and with a swipe of her wand she began to duel. Harry watched with terror and elation as Molly Weasley’s wand slashed and twisted, and Bellatrix Lestrange’s smile faltered and became a snarl. Jets of light flew from both wands, the floor around the witches’ feet became hot and cracked; both women were fighting to kill.
“No!” Mrs. Weasley cried as a few students ran forward, trying to come to her aid. “Get back! Get back! She is mine!” Hundreds of people now lined the walls, watching the two fights, Voldemort and his three opponents, Bellatrix and Molly, and Harry stood, invisible, torn between both, wanting to attack and yet to protect, unable to be sure that he would not hit the innocent.
“What will happen to your children when I’ve killed you?” taunted Bellatrix, as mad as her master, capering as Molly’s curses danced around her. “When Mummy’s gone the same way as Freddie?”
“You — will — never — touch — our — children — again!” screamed Mrs. Weasley. Bellatrix laughed, the same exhilarated laugh her cousin Sirius had given as he toppled backward through the veil, and suddenly Harry knew what was going to happen before it did.
Molly’s curse soared beneath Bellatrix’s outstretched arm and hit her squarely in the chest, directly over her heart.
Bellatrix’s gloating smile froze, her eyes began to bulge: For the tiniest space of time she knew what had happened, and then she toppled, and the watching crowd roared, and Voldemort screamed.
From the quote above we see Bellatrix duelling three witches at the same time and equalling all three.
Bellatrix killed Sirius who was an expert in dueling. Bellatrix defeated Aurors (who are trained specifically to duel death eaters) Frank and Alice Longbottom.
If your answer is that Molly is angry please consider it already states in the above quote:
"...taunted Bellatrix, as mad as her master"
So I'd like some in canon proof that being angrier than someone during a duel overrides their ability and experience.
If for some reason your answer is that Bellatrix is not focused on her new opponent, please consider in the above passage:
Bellatrix spun on the spot, roaring with laughter at the sight of her new challenger.
If it's something about being protective of her children, please explain why this would matter in a dueling situation like this, unlike Lily's which was clearly self sacrifice and explained in great detail.
How could Molly a witch hardly trained in dueling with almost no comparable real life experience in dueling, beat one of the most powerful witches in the series?