First, download the saddle up world, which I posted over here:Achievement worlds
Secondly, saddle the pig and ride him. Next, place the water which is in the bucket on top of the brick furthest away from the edge. The pig will stay on the edge of the cliff. Next hit him once and he should fall down, causing him damage.
More detailed explanation of how to get it:
the trench is 9 bricks/spaces long. On one end should be a small cliff. If you place water on the other end of the trench, water will flow from that end and it will stop right on the edge of the cliff. This will also make the pig stand right on the edge. If you ride the pig with the saddle and then hit him once(while you are riding him), you should fall down the cliff with the pig