LogFAQs > #943897261

To point out, Fiendfyre works on boggarts, Quirrel has an (unnamed) spell that squirts out magical acid that is heavily damaging and painful and worst comes to worst, Imperiusing Spinel into coming contact into his wand and being transfigured would probably work (unless she has some sort of latent magic resistance, but any shields which can be dispelled would probably be dispelled).

As for speed:

Harry could hardly see it, could hardly make out anything amid the lights and flashes, [from his position a few meters away] but the scene was still too small, and Harry had the feeling he wouldn't be able to understand even if he was watching from a meter away, it was all happening too fast, red blasts deflecting from blue shields, green bars of light clashing together, shadowy forms appearing and vanishing, he couldn't even tell who was casting what, except that the Auror was shouting incantation after incantation and frantically dodging while Professor Quirrell's Polyjuiced form stood in one place and flicked his wand, mostly silently, but now and then pronouncing words in unrecognizable languages that would white out the whole mirror and show half the Auror's shielding torn away as he staggered back.

Harry had seen exhibition duels between the strongest seventh-year students, and this was so far above it that Harry's mind felt numbed, looking at how far he had left to go. There wasn't a single seventh-year student who could have lasted half a minute against the Auror, all three seventh-year armies put together might not be able to scratch the Defense Professor...

And also Quirrell essentially singlehandedly hexes and stuns an entire corridor full of 44 Hogwarts students, with several of them competent enough to become Aurors on graduation with numerous dueling accolades and, of course, Quirrell has obviously never lost a duel against a Wizard with two obvious exceptions which do not apply here.

Maybe Spinel is strong and fast, but actual reasons need to be provided.