Aim straight at the first metal block. The angle does not need to be exact because the timing of when you use the power up is more important.
Power Activation Point
Activate the power up when you are about a little less than half way to the metal block. You want to grab onto the metal block, swing it half way to trigger the TNT. For this stage, you can either release the grab or just go all the way to trigger the bomb.
First Shot Result
After setting off the bomb. All the metal blocks and chains drop down below. The blocks and chains will either kill the pigs directly, or destroy the structures which kill the pigs indirectly. You will get the three stars if you can pull it off in one shot.