Making enemys smart enough to open doors?

[Equil]

I have another question. Is it possible to make monsters be able to clip through entities? Sure this might be immersion breaking.. but I have a friend who just loves to hide up on bookshelves to escape monsters, or a makeshift tower made of boxes. The monster will just run up and mindlessly bash whatever he's standing on and then eventually wander off.

One solution I came up with was filling up a fair portion of the level with block boxes, but this is tedious to do and can restrict the player's mobility unintentionally.