Make a long tube of obsidian (works best with 3x3x6 or 3x3x7) with a hollow middle. Then, put two blocks of obsidian on the top. Hollow it out. Fill with TNT, and put an nonblast resistant block in the last space on the front. Cover the top with redstone, and place a button on the back. Optionally, make a platform to stand on when you trigger it.