Working on a scene where a character is repeatedly struck down by having objects thrown at her, starting the scene with the sound she would make after being hit with a ball made of solid ice.
Assuming you mean the vocal sound the person makes, then perhaps oof. From Oxford Dictionaries:
Expressing discomfort, as from sudden exertion or a blow to one's body.
he gloated, as he formed a large icy snowball in his fat fingers. Then he said, ”Oof!” A large, hard snowball had caught him in the stomach. (Merv Lambert, Beware of the Emu!, 2012)
I think you're looking for dialogue, but if you're looking for narration you might still be able to use this.
It is also occasionally used as a verb, especially in indirectly reported speech:
“I don't—” she began, then “oofed” as the wraith suddenly became corporeal and shoved her back a few feet. (Scott Ciencin, Age of Unreason, 2012)
Finally, it might possibly be echoic enough that you could also use it for the physical sound of the ice hitting her body, especially if she's wearing a puffy coat.