The See-Saw encounter takes place on, you guessed it, a gigantic See-Saw over a deep pit. Our heroes are deposited on one end of the See-Saw, and enemies (of equal weight), are placed on the other end. Our heroes must defeat their enemies, but take care not to overturn the entire device, sending everybody on it tumbling to the hard ground below.
Setting up the Encounter
In general, you want to place enemies with a similar composition to the party on the other side - medium creatures, a mix of ranged and melee, and the same number as there are heroes. Feel free to deviate from this, but not too much. If you place four ranged large creatures on one side of the See-Saw, then it is almost guaranteed to overturn within three rounds. Also, you are probably going to want to use constructs or undead - who else would just be hanging out on a gigantic See-Saw?
Balancing the See-Saw
At the start of each round after the first, tally the weight on each side of See-Saw. If one side has more weight than the other side, the See-Saw tilts in that direction, and is considered "Tilted." Tilting the See-Saw in the opposite direction results in it become "Flat" for the next round. If the See-Saw is "Tilted," and then tilts in the same direction, it becomes "Severely Tilted." One more tilt in that same direction, and the See-Saw over turns, breaks, and all character fall to the ground.
Let's have an example. Round One: The See-Saw is flat, as it will always be for round one. Round Two: More weight is on the West side, so the See-Saw is "Tilted" West. Round Three: More weight is on the East side, so the See-Saw goes from "Tilted" West to "Flat." Round Four: More weight is still on the East side, so the See-Saw is "Tilted East." Round Five: More weight is still on the East side, so the See-Saw is "Severely Tilted East." Round Six: At the start of this round, more weight was still on the East side, so the See-Saw overturns.
Any round that the See-Saw is "Severely Tilted," all characters on the See-Saw suffer a -2 penalty to attack rolls, skill checks, and concentration checks. Spell Casters must make a DC 10 + Spell Level Concentration Check (vigorous motion) to cast spells. Additionally, all loose items (such as dead bodies) roll 4 squares downhill. Any bull rush attempts downhill gain a +5 bonus, and any bull rush attempts uphill incur a -5 penalty.
If the See-Saw ever goes from "Tilted" to "Severely Tilted," all characters on the See-Saw must make a DC 10 acrobatic check or fall prone and slide one square downhill (if possible).
If the See-Saw overturns, then all characters on the See-Saw instantly fall down to the bottom of the pit and are dealt the appropriate fall damage. Those on the lower half of the See-Saw fall 30 feet, those on the higher half fall 70 feet. They must then make an immediate reflex save to dodge the pieces of See-Saw as it comes crashing to the ground. The See-Saw pieces deal another 5d6 damage, but a DC 18 reflex save halves this damage. After the battle is over, the survivors can climb up a ladder to the exit of the room.
After the battle, if they have not fallen, our heroes will find themselves on a still swaying See-Saw. There are a few ways to get to the exit without overturning the See-Saw. They can simply all run across, and hope that they all make it before the See-Saw overturns. They can drag some dead bodies around to counterbalance themselves. Or they can tilt the See-Saw in the wrong direction, then mosey along over as the See-Saw rights itself. In any case, it shouldn't be too difficult, although looting the bodies may take a little bit more thinking.
Optional Rule - Torque
In real life, we would have to use a complicated formula to determine exactly how much force each weight exerts. We can get a little bit closer to that, but not give ourselves a headache, by factoring in distance from the fulcrum. Simply double to effective weight of any person more than 5 squares from the fulcrum. If you use this rule, you may want to tell your players about it.
Optional Rule - The Monster at the Bottom
If you want to spice things up even more, you can add a monster at the bottom of the pit. This monster will attack both parties if the See-Saw falls. It should be big, imposing, and strange - perhaps a tentacle monster or eldritch horror. Scatter some skulls around for good measure, and be sure to scare the heroes should they see it. However, the monster should have no ability to effect the fight above, nor bring the heroes down should the See-Saw remain stable.