Do-Ten-Kai-Zan

Do-Ten-Kai-Zan (ドテンカイザン) is the Four Generals' ultimate Gunmen, a merger of their four individual Dai-Guns. It appears only in the first film adaptation of the series, Gurren Lagann The Movie: Childhood's End. The full-presumably considerable-power of this Gunmen is not truly seen as it quickly is dispatched by the Dai-Gurren and Gurren-Lagann within moments of its appearance.

Formation
Formed from the Dai-Gunzan, Dai-Gunkai, Dai-Gunten, and Dai-Gundo, this mecha is the absolute fortress of the Four Generals. However, with Dai-Gunzan having been taken by Team Dai-Gurren, and its General Thymilph killed, the Dai-Gunzan-Du piloted by Viral served as a substitute. To begin the merger, Dai-Gundo spins rapidly as a rainbow-coloured cloud engulfs the Dai-Guns. Dai-Gundo then splits in half and attaches to either side of Dai-Gunzan-Du's hull. Dai-Gunkai splits along its length and attaches to Dai-Gunzan-Du's upper pair of arms. Dai-Gunten forms into an 'X' shape, as the three other Dai-Guns lower down onto it. The final plates slide into place, and Do-Ten-Kai-Zan is complete.

In Childhood's End
After Cytomander was killed by a newly confident Simon, Adiane, Guame, and Viral retreated to their respective Dai-Guns, and united them into Do-Ten-Kai-Zan. The super-sized flagship launched a fearsome barrage of missiles at Dai-Gurren, but Simon's shield of Spiral Energy protected them. Team Dai-Gurren then used the powerful "Giga Drill Break: Team Dai-Gurren Special" attack, which tore through Dai-Gunten, Dai-Gundo (killing Guame), Dai-Gunkai (killing Adiane), and finally Dai-Gunzan-Du (narrowly missing Viral), destroying the ultimate Dai-Gun. Viral was the only survivor, his vengeful feelings intensified even further by the defeat.

Due to Cytomander's worldwide broadcast (having expected an execution to further cow humanity), the destruction of the Do-Ten-Kai-Zan was broadcast around the world, and inspired humans across Earth to take to the surface themselves.

Attacks
In its short appearance, Do-Ten-Kai-Zan makes use of all four Dai-Guns' missile ports to unleash a massive barrage of artillery. It is a tremendous attack that covers all of the mecha's blind spots, but was unable to harm the Dai-Gurren.

It could presumably launch fleets of Gunmen from Dai-Gunten's hangars and use its arms and massive bladed hull for lethal physical attacks, but never got a chance to use them, as Team Dai-Gurren eliminated it immediately after its first failed attack.

Trivia

 * This Dai-Gun was made to cut on time since the first movie, Childhood's End, is a compilation of the first 14 episodes. It replaces the individual combats with the remaining Supreme Generals.
 * It is unclear how many Generals need to be present for the formation of Do-Ten-Kai-Zan. According to the movie, at least one can be absent (Cytomander's Dai-Gunten combines with the rest, despite its commander being dead). Also, before intiating the combination, Adiane points out to Guame that there are only two Generals left, implying that at least three Generals (or equivalent Dai-Gun commanders) must be present to transform the Dai-Guns.
 * The rainbow-coloured cloud generated by Dai-Gundo to protect the combining Dai-Guns may be an homage to the Final Fusion sequence in another Super Robot anime, King of Braves GaoGaiGar. It may also be a reference to the spiral forcefield Guame's Dai-Gun creates to protect Teppelin in the fourteenth episode of the anime.
 * The destruction of the Do-Ten-Kai-Zan calls back to the destructions of its component mecha in the anime: Dai-Gundo is pierced from below, Dai-Gunkai is shredded from within after swallowing Gurren Lagann, and Dai-Gunzan-Du's control tower is pierced by a drill, which narrowly misses its commander.
 * The official subtitles accidentally mislabel the Do-Ten-Kai-Zan as a 'Dai-Gan', reflecting on the Japanese pronunciation of the 'Gun' in 'Gunmen', 'Dai-Gun', etc.
 * The name "Do-Ten-Kai-Zan" is a combination of the last syllables of the Four Generals' individual Dai-Guns: Dai-Gundo, Dai-Gunten, Dai-Gunkai, and Dai-Gunzan (or in this case, Dai-Gunzan-Du).