Minecraft wiki entry for Cave_spider_farming, associated text: In this design, there are two different modes. In one, the cave spiders take lava damage until down to half of a heart, when water puts the lava out. This allows for players to kill them in one hit. In the other, the cave spiders are killed completely. This design is meant for 3 cave spider spawners, but could easily be modified to 1 or 2.