Arya VIII, Storm
We know prophecies don't always turn out quite the way we, and the characters, expect. So we should expect a twist here. There is no doubt that the maid with purple serpents in her hair is Sansa and that vision alludes to Sansa's role in the assassination of Joffrey. And since the woods witch saw the same maid slaying the savage giant it's usually taken as a given that Sansa will do the slaying. While some suggest the giant is Gregor, and some suggest it's Tyrion and a few even have some off beat notions, most seem to agree that the giant is Petyr and Sansa will whack him in Winterfell or maybe the Eyrie. And we want that to happen because Sansa is so sweet and Petyr is so dastardly. It would all be so neat.
But whom did the woods witch see? Was it really Sansa or could it have been someone who looked like Sansa? Perhaps her sister? Well, Arya doesn't look like her, does she? But Arya is learning how to look like anyone she wants to.
And we know the George just loves sets of three. In the first vision we get from the woods witch, we learn about Stannis killing Renly, Euron killing Balon, and Catelyn getting whacked. Arya IV, Storm. In the second set above we get Rob at the "red wedding," Sansa at the purple wedding, and... Sansa again? Really? Wouldn't the sets of prophecies from the ghost of High Heart be more elegant if the maid again was someone other than Sansa, perhaps Arya, maybe even at a wedding?
And notice that as soon as the woods witch tells us that the maid again will slay the savage giant in the castle of snow she turns her head sharply and smiles through the gloom, right at Arya.