Nimbus Rod

Cast it and a storm cloud parks where you aimed and rains on everything underneath for several minutes. Only the cast itself costs mana, and you are free to fight with any other weapon while it works, so all that damage is effectively free once it is up. It behaves far more like a turret you placed than like a spell you are maintaining.

You get two at a time, and a third cast pushes the oldest one out. The cloud cannot travel through solid blocks or a closed door on its way to the cursor, though platforms and water are no obstacle, so cast into open air above where the fight will happen and then move underneath. In a built arena that means seeding both clouds over the middle before you summon anything.

• Critical chance is taken from whatever you are holding at the time, not from the rod. Cast the clouds, then hold a high crit magic weapon and the rain inherits it.
• Every individual drop counts as a hit for armor bonuses that trigger on landing damage, so those sets stay lit constantly with no effort.
• The drops pierce and fall in a dense column, which is why this is a standard answer to the Destroyer and to anything else long and segmented.
• The pre-Hardmode crimson version counts separately, so keeping it means three clouds on screen at once.

The enemy that drops it only appears in rain and is not common even then. Build a small covered spot you can shoot from safely, run spawn boosting gear through a storm, and let it come to you rather than chasing it across the surface.