Um, Dora's head would probably smash Elmo before he could do anything, but then again, she'd probably ask the kids 'I need something to hit Elmo with, do you see something to hit Elmo with?' andthere's be a sledgehammer right next to her feet, or she'd go into her backpack and pull out something unreasonably large that couldn't fit in it by normal physics, guess that's good, no matter what it is it can fit, it will fit. Elmo would probably talk about it for a little, explaining that this is not something you should do but it's okay because Elmo knows best, then he'd attack her with songs.
So which would win in my view? Neither, they'd both kill each other but Elmo will die first. Then Dora will have a slow death by all of Elmo's songs stuck in her head. Which will explode or something :\\