Writing Luna
If you haven't noticed Luna is somewhat off. Buffy fans might be tempted to write her as a blonde Drusilla, don't be. Luna makes sense when she talks, she just says it from a different perspective than the rest of us.
"They're spirits of fire," said Luna, her protuberant eyes widening so that she looked madder than ever. "Great tall flaming creatures that gallop across the ground burning everything in front of...There are plenty of eyewitness accounts, just because you're so narrow-minded you need to have everything shoved under your nose before you-"
What she says isn't that weird, she just has some odd beliefs. You can have her having a conversation without talking nargles every five seconds. She doesn't copy herself. Don't make her copy herself, she an interesting girl and can find new things. Unless you sort of bring the conversation around, she isn't going to say outlandish things. Her behavior is sometimes different from what you'd expect. Like when they were captured by Umbridge and the Slytherins everyone else was struggling she stayed still and didn't get choked like Neville. She seems to have a serene personality and nothing bothers her, but that just may be an act to show all her tormentors that they don't bug her. Everything she does has a purpose. Luna is a multilayered, character not some girl who says completely odd things every minute.