I think this was a villainess novel. Maybe.. The MC had ice magic I think. At the very least she only had one element. Part of the storyline is dealing with demon attacks. The MC focuses on increasing her strength and magic to prepare for the invasion. I think the demons end up attacking faster, or are stronger than what she was anticipating according to her knowledge. Not much to go off of, but most villainess aren't worried about demons I think. Sorry I dont have more.
Is it an apocalypse one? If yes then maybe Dominion's End. MC is male, but has memories as a female who lived during the apocalypse (he doesn't remember who he is, just her memories) and awakens ice magic. Edit: Oh, wait no demons so probably not. Still it's a good story.
"I Reincarnated as a Noble Girl Villainess But Why Did It Turn Out This Way?" I don't think it's the one you've been looking for because the MC don't have ice magic, but it's still interesting and the MC battle and assume the role of a boy.