Creeper

You never summon these separately. A swarm of them arrives with the Brain of Cthulhu and orbits it in a rotating shell, and the boss takes no damage at all while any of them are alive. They do not come back once killed, so the opening is a clearing job with a fixed end point rather than a fight you can lose ground in. For the Worthy worlds double the swarm, which changes the pacing of that phase completely.

They cluster tightly by design, so anything hitting more than one target does several times the work of a single target weapon. Piercing ammunition such as Jester's Arrows or Unholy Arrows earns its place here, and a Grenade thrown into the orbit clears a group at once. Hold to one side and let the rotation carry them through your line of fire instead of chasing individuals around the boss.

Creepers drop hearts far more readily than ordinary enemies once you are already hurt. In practice the swarm phase is where you top yourself back up for the second half, so do not blast through it untouched and then start on the boss with nothing left on the field. Save the potions for later and eat the hearts now.

Contact stops being a plain hit. It always applies a debuff, pulled from a list of nine, so expect to be blinded, slowed or poisoned before the boss is even targetable. Build the arena so you are not depending on precise movement while one of those is running. One small mercy: unlike most enemies in those modes, these cannot pick up the coins you drop.

In Classic the boss itself gives no Tissue Samples at all, only the swarm does, so the ring is the actual source of your crimson armor material. Expert moves part of that into the treasure bag instead. They drop no souls and no biome keys, so a creeper farm replaces nothing else you might be grinding.