The PCs face off against the wave of monsters and cannot let them enter the final row of the Gauntlet room (or other goal area).
An actual Gauntlet defensive line would be very difficult if the room were 10 squares wide as the party is likely 4-6 characters strong. They would have to rely heavily on OAs to prevent forward progress by the minions.
More likely is an assortment of regular monsters and minions that work to progress through the room. Monsters with teleportation and flight would be rude at low levels but might be an appropriate challenge at higher levels. Powers that slow, immobilize, and push/pull/slide would be able to shine in such a fight.
A variant could include defending a burst 1 area from enemy occupation. The game could end if an enemy starts its turn in the area or points could be deducted from a set total for each such occurrence with the goal being longevity.
A team of dwarves could advance upon the area and make it very difficult to remove them. Lurker monsters could constantly spring attack their way into the area without provoking. Teleporting/flying monsters would challenge players to utilize forced movement powers. Points should be lost for PCs standing in the area.