In fantasy, encounters are best when there is some complication, a dangerous location, for example, difficult weather, precarious ledges, rough terrain, bottlenecks, traps, weird devices PCs can take advantage of, etc.
In Horror, on the other hand, the best encounters are the ones where one or more of the abilities that the character have come to rely on are rendered useless. Fear, entanglement, being disarmed, dealing with incorporeal creatures, mind control, anti-magic (or better yet wild magic) zones.
Don't do this every encounter, of course, and certainly don;t target a single PC more than once, the trick is not so much that you are trying to 'beat' the PCs by taking their power away, as putting them on unpredictable, confusing, or uncertain ground in a battle. When they are stuck in an alien environment or where the 'rules' (rules of reality, not the rules of the game) seem to have changed.
Also, make sure to destroy any sense of security they might have: the safe sealable rooms in the dungeon they might arcane lock to get some sleep should be the ones with secret doors in the back of them. They should have 'presents' or messages delivered to them by unseen hands despite all attempts at security.