This is mostly for new players on their first playthrough who are having trouble during the fight. This isn't a tutorial on how to spawn it.

I would recommend gold or platinum armor, and try to get a platinum/gold bow with jester arrows. Jester arrows go through multiple targets, and Brain of Cthulhu spawns multiple minions, so, y'know. Hermes boots as well as a (Whichever works) in a bottle are also useful.

You will want to place platforms and torches within the main cavity of the crimson cave. the shape of it is an orb, just big enough for an arena. make sure you have multiple rows of platforms, that you can jump between easily. You'll want an ironskin and swiftness potion at minimum. So drink your potions, break the last heart, and get ready to fight.

When the fight begins (Some banger music will play) The brain will spawn many "Creepers" to attack. Your jester arrows should shred through these. When all Creepers are down, the brain itself will attack. Keep your distance, and try not to run too fast in the opposite direction of the brain, as it may teleport directly where you're running.

Assuming you managed to defeat the brain (Nice job, by the way) collect what it drops. The creepers also dropped loot, so make sure to get that in case it fell down a chasm. With this loot you can make new weapons and armor, so you'll be prepared for the next boss. I hope this guide helped, feedback (Hopefully good) is appreciated. Good luck out there.