Kirby Super Star Ultra screenshot
The Reactor is the Halberd's main power plant. It is a large room where Wheelies continuously run within chambers to generate power, but the part of it that is fought looks like the central conduit of some sort, with a floating crystal shaped like a regular octahedron in the middle that resembles a single rainbow Pix.
Kirby Super Star and Kirby Super Star Ultra
In Kirby Super Star and Kirby Super Star Ultra, the reactor is guarded by a set of auto-defenses, including a pinpoint laser gun on an extendable arm, a large pulse cannon, and a series of burners under the floor. According to Sailor Waddle Dee, the Reactor is invincible unless it shoots itself (via the laser gun), making its defenses its Achilles' heel. Since the laser aims at Kirby/Meta Knight, flying next to the reactor's crystal and then quickly getting out of the way of the crystal is an easy way to defeat it.
While all of the auto-defenses can be destroyed, it is often impractical to do so, as only destroying the core is necessary to win. The pulse cannon has high enough health that there would already have been enough laser beams fired to destroy the Reactor by the time Kirby gets rid of it, unless Kirby is wielding a powerful ability that can destroy it within 3 cycles. Meanwhile, the burners can only be destroyed by attacks that reach below the ground. The burners and laser gun will regenerate, though the pulse cannon will not.
Kirby's Epic Yarn
Kirby encounters the Reactor in yarn form in Kirby's Epic Yarn while traveling through the Halberd on the Star Shooter. Unlike Kirby Super Star (Ultra), it is no longer invincible, and Kirby (or Prince Fluff) has to destroy it by firing on the central crystal himself. The laser gun, cannon, and floor burners are absent, but it is now defended by two long-range flamethrowers - one at the top and one at the bottom. These are defensive as well as offensive - yarn star projectiles from the Star Shooter cannot pass through the walls of flame. Also, the Reactor's defenses are no longer capable of shooting itself. UFOs join the fight every now and then to fire laser beams at Kirby.
Wheelies are not seen in the background of the Reactor in this game, so perhaps the methods of power generation of the Reactor had been changed to a different method.