To explain the final point in more detail -
You completely defeat a horror by using food it loves.
Upon doing so, a little marker of some sort will appear in the bestiary on the horror's page.
When that mark appears, there is a chance (I'd say something like 1 in 5 or 1 in 4) that the horror will recognise the princess on an encounter.
When they do, they stop advancing and you can simply keep feeding them and yourself food until the encounter is over.