Minecraft wiki entry for Mob_grinder, associated text: Not all mobs can be easily killed by magma blocks. Nether mobs, such as wither skeletons and blazes are not affected by fire damage, so magma blocks are not useful as a nether grinder. Magma blocks are also ineffective against witches as they drink fire resistance potions negating damage. Spiders can also be tricky as they may climb walls and cause issues.