Isn't it interesting how many of them have imp- in them? That's what slowed me down on easy ones (immune, imagine)-because I was thinking of imp- words-not im-.
I would have got two more correct had you accepted other forms of the word. as defined on the wikipedia page for Improvisation. "Improvisation in the performing arts is a very spontaneous performance without specific or scripted preparation" that sure seems to match your definition of To perform impromptu. I would also think Immolation could work too as it's just a different form of Immolate. good challenging quiz that I enjoyed.