You're going to need to kill a lot of creepers...
1 Sticky Piston
5 Slime Blocks
192 (or more if wanted) TNT
Step 1: How to Make
Just as an FYI: I don't take pictures of each individual step in the game, I draw them out on a drawing software.
You need to add a slime block to the right of the dispenser if looking at it from the left side.
Step 2: Add TNT
Put 3 or more stacks of 64 TNT into the dispenser for mass-destruction!