For a low-mid-level party, put a spellcasting bad guy on top of a long staircase. (I used a step-pyramid with only one easy route up). Make him big on Summon Monster Of Appropriate Level spells, and/or speckle the staircase with low-level goons. Climbing stairs costs double movement (house-ruled on the spot), so the PCs are slowed to a veritable crawl as they cut their way up the stairs, hustling before his direct-damage spells start dropping people.
I also put a big burning brazier up at the top of the pyramid, something for him to hide behind (giving him Concealment) while he cast summon spells, but I think that part's optional.
This worked suprisingly well for me. The PCs made it to the top at low hit points, the sorceror started desperately hurling damage spells, PCs started falling over, and then they finally pinned him into melee and tore him apart. Much fun was had by all.
Anyone else got any tips?