If you have one illegal cockfight, that's singular. If you have two illegal cockfights, it's plural - though where's the "of" in that example?
"Stuffs of nightmare/s" is never used. The only possible phrase is "stuff of nightmares" - because it's the stuff nightmares are made of - nothing to do with "of".
With the first phrase, you're dealing with whether "experience" is countable or uncountable.
Student or Learner