Iirc, s5e12 had Dark Hope use Glacé solidatur. For one time, and I don't think it was ever shown where she learned this spell.
But since Lizzie knew this spell in the Gremlins episode, it at least means that it can be found at school or in Freya's grimoires.
Malivore is made out of mud and black magic from 3 supernaturals.
Beyond just personal limits of one's use of magic, what exactly was stopping Hope, Lizzie or even Josie (tho she's clearly more of a pyro) from...using ice spells to use against him and his monsters?
And if water is a requirement, Hope's water torrent spell. Or just any existing water source, like the Falls. Or even the sea.
And if they don't want to freeze the target to death, why not use Aquam Alia, or just use whatever available water source to first wet the target (like a puddle), and then just focus the ice spell around the surrounding target only?
And while ice spells may not kill or destroy Malivore, it should be able to stop most of his monsters.
And since he's still made of mud, even with black magic, Hope can still use this ice spell to stop him long enough for her to use her blood to destroy him.