Keep em' coming! :D

I suspect you could have a MMM inside another MMM (depending on the wording of the rules). If that's possible, there are numerous possibilities.

New brainstorm: Create a bunch of rooms, half of which are bricked off and require teleportation magic. In fact, include doorways that are simply bricked off (there is no room on the other side) so when they figure out that they can teleport, it will fail some of the time. Furthermore, hide in a room with an open doorway, but use silent image to brick it off. The BBEG will be so flustered with the other doors, he will likely see the illusion and not even bother to interact with it.

At least, that will work until he starts blasting holes in walls (which is still good, because then he is wasting his spells).