Guide:Brain of Cthulhu strategies

Fighting the Brain of Cthulhu is very similar to fighting the Eye of Cthulhu, with a little tweak on phases. If you have the basics of how to Fight the Eye of Cthulhu, then you will have a better chance fighting the Brain.

Terrain Preparation
You'll want to set up a long platform in The Crimson, because it can be summoned there.It can be summoned in caverns and above ground It should be high enough so ground enemies can't jump up to you, but low enough that you don't accidentally go high enough to make it despawn. It is similar to the Eater of Worlds in the way that it can be summoned and fought during day or night. An arena the size of your screen is preferred, since you can see the whole area in the middle, it is safe, and enemies can't go jumping in.

If you feel confident in the crimson, and don't mind the possibility of other crimson enemies barging in on your fight, you can set up a series of wooden platforms inside the large, circular hollows of the underground crimson. These spaces are usually near 6-10 hearts, giving you a chance to kill 2 or 3 brains in a row using terrain prepared on-the-spot. You can grapple up to wooden platforms, so you can space the platforms up or down more than your normal jumping range if you have a Grappling Hook, already. (Even when holding down, you fall faster with less platforms.)

Keep in mind that the boss will spawn WHEN YOU SMASH THE THIRD HEART, setting you immediately into battle, not when you take your time getting to the arena. Have a path to your arena ready, and try to save any hearts close to your arena as the third heart to smash in a series unless you want to build more arenas every time you want to fight a new Brain.

Armor
If you want to stand a chance against the Brain of Cthulhu, you're going to need at least Iron or Lead armor. Of course the better armor you have, the easier the fight will be. If you have access to Platinum armor, be sure to make it, as you benefit from it greatly.

Weapons
As for weapons, it is recommended to use ranged or magic to avoid taking unnecessary damage.

Bows with Jester's or Unholy Arrows are good for their high damage. Because they are piercing weapons, both arrow types can hit multiple eyes. However, one can also use Flaming Arrows if they wish to. Recommended bows are the Platinum Bow or the Gold Bow. Of course, if you can make the Tendon Bow or the Denom Bow, it is advised to take advantage of it and make it.

For guns, you might want to find an Undertaker from Demon Hearts, the equivalent to the Shadow Orb in The Crimson. Also make sure to buy enough musket rounds.

In the beginning of the battle, Grenades are highly useful against the clumps of Creepers that form. Only a few are necessary to take many of the Creepers down.

For thrown, get lots of Shurikens or Throwing Knifes. Also, the Harpoon seem to work great under its second face, because of the great knockback. The harpoon is dropped from the goblins, so that means you will need to fight a goblin army first to get this.

As for the Magic users, craft a Sapphire Staff or better when fighting him, and get at least 100 mana if you don't want to continually run out and hope that one of the orbiting eyes drop another star. If you have already beaten the Queen Bee, the Bee Gun is useful to get rid of the Creepers. Also, if you have spare Stars, Bottle, and Gel, then you should make some Lesser Mana Potion, since magic users will need a lot of mana to fight the brain. If you fought Skeletron somehow, then go into the dungeon, as the weaponry in there will absolutely wreck the Brain. The Water Bolt can demolish him if you have the mana (around 200 for it). The Aqua Scepter is even better since you have decent knockback, infinite piercing, and low mana cost, but that means you need a locked golden chest first. If you found a Floating Island, then make use of the equipment you get. If you're lucky, you can get the Starfury and make it an easy fight.

If you have access to a world with Corruption, the Vilethorn is tremendously potent against the eyes in the first half of the fight - the Vilethorn can penetrate infinite enemies, and has enough knockback to stun the whole crowd of Creepers, making the enemy evaporate. It's also light on mana if you space your shots out. (With 200 mana, you can regenerate all mana spent on the Vilethorn before the thorn effect, and its knockback, ends, giving you excellent juggling capabilities.)

The second phase of the boss is similar to the Eye of Cthulhu, but the teleportations can sometimes make the boss easier, as it tends to make the boss back off from the player a little.

It may be best to switch to a weapon with higher one-target damage against the brain - you will likely be shooting at it from fairly close range, and there will only be one target, so your choice of weapon may change. Fire arrows on the strongest bow you can bring, or The Undertaker will be more effective against the single, large target you face in the second half. If at all possible, defeat the Eye of Cthulhu first, then use the crimtane you get to make a Tendon Bow, then make fire arrows and watch in awe as the boss gets wiped out in about 2-3 minutes after it enters it's second stage.

Potions
About 5 Healing Potions, 1 Ironskin Potion and 1 Regeneration Potion

Accessories
Cloud in a Bottle (optional, but it helps when he lunges). Band of Regeneration (optional, but every health point counts). Aglet (optional, but usually anyone can get one easily and +5% Movement Speed helps). Cobalt Shield (If you went to the dungeon, then this is a great addition to anyone since knockback is annoying).