I'm surrounded by idiots. Who do I have to kill to get my hands on a superior brain?
Solution 1:
Every monster has a 7% chance of dropping a Superiour Brain instead of an Inferior Brain when killed with a Brain Extractor. Just keep killing monsters, and you will get one.
Solution 2:
From my own experience it's just a rarer drop than the inferior brains. They can drop from any monsters killed by the brain extractor.