I've always been fond of illusions, and multi-layered illusions are wonderful things. An illusion of a labyrinth (including floor) over a pit trap is very similar to the one above, but add another layer. The stairs out of the pit trap along with the monster at the end are the only way out - disbelieve at your peril. Okay, they're going to get out, but it'll make it harder. Then there's the old standbys, like casting an illusion of a hydra over a basilisk. Go, on, stare at the hydra and say you disbelieve...