Balls-O-Tron

Background
Balls-O-Tron is a boss resembling a gargantuan mechanical worm. It will drop the Balls-O-Tron Coin upon death, similar to Mask Man.

Spawning
When spawning Balls-O-Tron for the first time, you will need a Large Silver Key and a mechanist's circle (which can be found in a jungle dungeon).

To summon it, you have to use a Large Silver Key on the circle.

Another way to summon Balls-O-Tron is to use his spawn item, Balls-O-Tron Prime.

Behavior
Balls-O-Tron spawns above the circle it was summoned with. It dives into the ground, trying to burrow under the player. It's head deals extreme amounts of damage, so avoid being on the ground for too long.

All of it's segments shoot green laser beams that deal a lot of damage, yet move quite slowly.

It behaves weirdly in creative mode, possibly because it cant find a target. May kill himself because of that occasionally.

Killing
Since running away from BOT is a viable strategy, a mobility set (like Lunar Cybernetic Armor) will work. It'll also help dodging the segments' lasers. Alternatively, using Schrabidium Armor allows you to minimise the laser damage to just half a heart, allowing to engage in melee combat.

In terms of dealing damage, the best weapons (for this stage of the game) are the Defabricator, the Nightmare Revolver and... pretty much everything you can stuff an IF R&D round into. You'll need more than a hundred of those rounds, so get as many Maskman Coins as you can. Also, nuking it is always an option, either with mini-nukes or something bigger.

Loot & Use
Balls-O-Tron drops a single Balls-O-Tron coin.

The coin is used to make the Fab Heart (and therefore the DNT nanosuit), the Martian Invasion Ship and Gerald. It can also be used to make Balls-O-Tron Prime, the spawn item for this boss.

Trivia

 * Balls-O-Tron is based on The Destroyer, a boss from Terraria.
 * Balls-O-Tron was canonically created by Mask Man as a mechanical penis enlargement, but was too big and gained sentience.