How do they know what the place looks like? Even if they've seen the place, redecoration may make the rooms very difficult to describe. Ex. move around the tapestries, and do a little painting. Also, why dosn't the bad guy have some sort of protection from scrying? If he can be scried, then he is vulnerabe to this trick. Or if he can detect scrying, he should be able to keep anyone from getting a good look, or spring a trap. When he detects the sensor, he moves his big allies to his current location. I think a leneran pyrohydra is a lot more dangerous if you teleport into a small locked room with it, than at about any other time.
However dimension door might be a bit more difficult, as it can be done without sight.