LogFAQs > #941870627

So to point out some things:

Quirrell can't change someone else into a cat or anything, transfiguration isn't arbitrary and free transfiguration (changing anything into anything else) requires contact between the wand and the target..

Quirrell might be able to get the jump on her, he has various invisibility spells + stealth mechanics (it's not exactly specified what they are, just makes him hard to detect both physically and magically), I think if he knows he has an adversary, it would be extremely in character for him to not engage unless he has a gigantic unfair advantage.

Other than the killing curse, he also would hex people 50 ways in the space of a second (he knocked out basically all of slytherin house alone, including several auror grade students without anyone noticing except by inference). A duel between Quirrell and a super experienced auror was described as like a rainbow of curses, counter curses and barriers etc.

It's pretty hard to say what he'd use other than the killing curse, except maybe like, dunno guns or whatever's the most effective (most effective assuming that it doesn't depend on anyone else). He's very straightforward, and what's terrifying is basically he decides not to murder most of the cast at the start.