In the example with monsters, perhaps the issue is that all the different monster types are classes instead of data. For instance, I would instead implement something like

  class Monster
    attr_reader :type
    def initialize(type, likes = {})
      @type = type
      @likes = likes
    end

    def likes?(x)
      @likes[x.type]
    end
  end

  orc = Monster.new(:orc)
  troll = Monster.new(:troll)
  elf = Monster.new(:elf)
  elf_maiden = Monster.new(:elf_maiden, :elf => true)

  puts troll.likes?(elf)
  puts elf_maiden.likes?(elf)

If you have 150 subclasses, it is a code smell in the first place, and perhaps by addressing the root problem, issues like the one Steve mentions won't occur in the first place.