I decided to make this because I'm doing nothing at the moment.
A new girl named Holly has become a student at Hogwarts and soon after, sightings of a large rainbow butterfly appearing near the forbidden forest. Harry has noticed Holly has been disappearing at different times of each day and decides to follow her. He ends up following her into the forbidden forest, there he see her being surrounded by a dark energy and crying out in pain. But before Harry could see what was wrong, Holly transforms before his very eyes into her true self, The Rainbow Butterfly.
Harry, in shock, steps back right on to a stick, snapping it in two and getting the Rainbow Butterfly's full attention. Pulling out his wand, Harry prepares for a fight for his life.
Agreed. Harry likely loses before figuring out her weakness. Though possible to figure out, in character he's more likely to defend using magic shield or running away with the Firebolt to evade direct combat in hopes of getting lucky with a patronus or something else. I'm gonna go with the Rainbow Butterfly that could actually and realistically exist in the Harry Potter mythos.
Not to throw too large a spanner in the works here, but Harry has an invisibility cloak in his standard equipment and he's got the Imperius Curse. Holly can't detect invisible stuff and she doesn't resist mind manip.
Also, Holly actually did resist Griffon's Mind Manipulation. All the normal monsters instantly became Griffon's slaves by using the darkness in their hearts however Holly actually resisted him for some time and manage to leave and find a place to hide in the forest before she completely got taken over be his evil consciousness.
True, the first thing he does is probably run away. At which point he's got an invisibility cloak. He can and has used the Imperius Curse in character, and there's also that other time he tried an unforgivable curse in a fight, so he can definitely attempt it without it being an OOC move. Unlikely to be the first thing he tries, but between an invisibility cloak and being able to set up some very effective area concealment/ protection spells, he's going to get around to it eventually.
If he can pull it off. He's going to turn around and run while reaching for the cloak which is a bad move because of Rainbow Butterflies ranges beam which is wide enough to catch him as he turns his back.
When did he use that curse again, can't remember. Regardless it won't work because Griffon's already controlling Holly's mind and Griffon's Mind Manipulation far exceeds Harry's.