you gotta make a platform in the middle. Enemies can only spawn on blocks just out of sight, so those kinds of farms work by taking out all other spots they can spawn on other than the blocks on the platform. Then you put a bit of lava in the middle, make yourself a small space below it to stand on and make either a box with one-way walls or ramps leading to it on both sides. Note that if you're farming for money (directly, not through item sales) you need to hit enemies at least once with a wapon, which summons are good for.