I remember reading it when I was young so it won’t have been published before probably about 1999/2000.
There’s a witch, and the protagonist (who I think is a boy but I could be wrong) has to vanquish her somehow, and she can only use earth, air, fire and water as her weapons.
When it’s air, she creates this deafening, howling wind, and as soon as he steps outside to face her it stops and falls completely silent. I think he’s got someone helping and guiding him, possibly an old man.
I have a vague memory of the witch being called Morgan Le Fay and thought it might be Whispering Knights by Penelope Lively but the plot summary of that doesn’t really match up. I don’t think it’s an Arthurian story but I could be wrong.
Any ideas?