Otherwise you have to wait for the lava at the top to drain out.Made by placing a wall then dropping sand on top of it, then when the enemy you want to kill is underneath, break the wall and let the sand fall on top of them.Warning: Sometimes this trap glitches while you are off screen.It is unknown what causes this but when you return the lava will have either overflown or disappeared completely.The lava will now pump out from the top and will fall back down to the inlet endlessly as long as the timer is active.Optionally you can add inactive/Active Stone Blocks to the hole at the top where the outlet is so that the lava will stop flowing faster when you turn it off, but attach it to something other than the timer.One of the nice features about this lava trap is that you can turn it off, allowing for safe passage rather than having to remove the lava manually.
It makes use of the new 1.1 mechanics so you will need a bucket of lava (or more depending on the height you are trying to cover), an Inlet Pump, an Outlet Pump, a Wrench and some Wire, and a 1 Second Timer.
Wires are not affected by lava so don't worry about damaging them.
Activate the timer either by right clicking it or attaching it to something else like a lever.
You can dig little nooks into the area where you want the wall to be so they don't stick out since players are able to walk over holes 1 block wide.
Attach both pumps to a 1 second Timer and add lava to the inlet pump at the bottom.
Sometimes lava will spill while falling, causing it to land on the ground next to where it was intended to and cause damage to the surrounding area.