Build a tall structure like this (the higher the structure the further you will shoot)
Build a small square at the very top of the structure.
Build an even smaller square coming off one of the corners
Build a small tube-like thing (this tube should be no more than 5 blocks heigh)
Now fill the tube with any mob (except endermen). There should be at least 100 mobs in the tube.
the mobs should be visible from outside the tube. Once this is done stand an the platform next to the tube and quickly run into the mobs.
You should shoot of the platform.
Step 8: Here Are Two Tips
Try pushing mobs into the launcher (the tube)
Or try making the platform slightly longer and see what happens