Minecraft wiki entry for Spawner_traps, associated text: Seal the zombie spawner in a 5×5×5 chamber, naturally, you want it to stay in the center of the chamber at the lowest height. Outside of the chamber make a platform that is 2 blocks high that surrounds the outer wall, and while standing on this platform, punch a wide hole in the corner of the chamber 1 block above the platform. Remove the corner block so you can stand in the corner while the zombies practically funnel toward you. They'll be able to push you, but not hit you at all, so you're free to smack them on the head with your weapon of choice.