Apologies to Elton John and the Disney Corporation on top of the usual JKR disclaimer.
"Harry, I've found the secret to solving all your problems!" Luna smiled dreamily at Harry as he enjoyed a day outside.
"Really?" Harry perked up a bit. "What is it?"
"Avada Kedavra," she sang happily as a green beam flew from her wand and slew a young Hufflepuff first year.
"Avada Kedavra!" Harry tried experimentally. He smiled as Draco Malfoy ceased to be a problem. "What a wonderful phrase!"
"Avada Kedavra!" Luna shot down Professor Snape, "Ain't no passing craze!"
Harry was getting into it and decided to sing along as he skipped arm in arm with her towards Hogsmeade, "It means no worries for the rest of your days."
"It's our problem-free philosophy," The sang together, gunning down Slytherins and random townsfolk.
"Avada Kedavra!" Luna cheered with a bit too much glee and not even caring who her spells hit anymore.
"Avada Kedavra?" A young boy asked curiously.
"Yeah. It's our motto!" Luna told him.
"What's a motto?"
Harry couldn't resist being a smart ass, "Nothing. What's a-motto with you?"
"Those two words will solve all your problems," Luna promised.
"That's right. Take Luna here," Harry gathered breath to continue his singing, "Why, when he was a young Ravenclaw..."
"When I was a young Ravenclaw," Luna's high note on the end shattered a window.
"Very nice," Harry complimented.
"Thanks!"
"She found her demeanor lacked a certain appeal," He switched to an upbeat tempo, "She could clear the Common Room with just a squeal."
"I'm a sensitive soul though I seem thick-skinned," She confided in the boy mournfully, "And it hurt that my friends always seemed chagrined."
"And oh, the shame," She wilted dramatically.
"She was ashamed," Harry added with zeal.
"Thought of changin' my name..."
"Oh but, what's in a name?"
"And I got downhearted..."
"Tell us how did ya feel?"
Luna looked shocked at the question but stopped singing to answer him, "Well I often fondled my breasts before I..."
"Hey! Luna! Not in front of the kids!" Harry clamped a hand over her mouth when he saw the boys eyes widen.
"Oh. Sorry," She looked sheepish and aimed her wand at the boy who knew too much and resumed singing, "Avada Kedavra! What a wonderful phrase!"
"Avada Kedavra!" Harry was surprised when Voldemort caught one right between the eyes as he portkeyed in with a group of Death Eaters. "Ain't no passing craze!"
"It means no worries for the rest of our days!" The surviving townsfolk sang cheerfully.
"It's our problem-free philosophy," Harry and Luna looked at the Death Eaters with glee and began chanting, "Avada Kedavra! Avada Kedavra! Avada Kedavra! Avada Kedavra! Avada Kedavra! Avada Kedavra! Avada Kedavra! Avada Kedavra! Avada..."
"It means no worries for the rest of our days!"
"It's our problem-free philosophy," Luna spun around happily, lost in the moment and never saw her wand point towards Harry as she geared up for the big finale, "Avada Kedavra!"
Read it again while humming "Hakuna Matata".