|Grock Type||Final Boss|
|Appearance||Dillon's Rolling Western|
|Stage Appearance||Stage 10 "The Last Stand" 3rd day|
|Weak-Point||Crystal Core (Inside the grock)|
|Defenses||Hovering, Core Pillars, Spawns Crysgrocks, Boss Spawns Diamongrocks|
- "The final boss, which differs from the other grocks in every way. Nobody knows when or how it was created, or why it is attacking."
Inside lies its core which is protected by pillars that glow, then spring up as part of the Ultimogrock's defense system. These pillars become faster the more damage is done to the Ultimogrock's core. Alongside these pillars, Crysgrocks and Diamongrocks, acting as antibodies, help to protect its core. After its defeat, the threat of the Grocks is completely eradicated. However, after completing The Old Mission, a teaser trailer plays for a second Dillon's Rolling Western with a return of the Grocks. This title is called Dillon's Rolling Western: The Last Ranger. This Boss is exclusively to Dillon's Rolling Western, in which it doesn't reappear in The Last Ranger.
The Ultimogrock never attacks directly, instead attacking Dillon from inside with Crysgrocks and Diamongrocks, acting as antibodies, in order to defend its core. It floats out of Dillon's reach, and can only brought earthbound if the player has equipped plenty of towers with guns. This is possible for as many times as you are able to ground it with weapons, plus a final opportunity when it touches the town and Russ throws the Victory Charm at it. Once on the ground, Dillon only has whatever time up to two minutes to enter inside it and do as much damage to its core as you can before it recovers and floats back up, repeating the whole process. If Dillon fails to defeat it during the time limit, a wave of energy will expel him from the Ultimogrock's mouth.
As would be expected of the last boss, the Ultimogrock's defenses are all but impenetrable from the outside. Its only weak point is a glowing egg-shaped crystal inside of it, which is also very tough. Making matters worse, the Crysgrocks and Diamongrocks that regularly spawn inside the Ultimogrock in addition to pillars that spring up around the core make it very difficult to attack the core to begin with. Exceptional timing will protect Dillon from the pillars and allow him to avoid them.
As the Ultimogrock's health whittles down, it will become increasingly desperate in its attacks with its pillars. The ones that protect the core will encircle it at first, then start springing up at random intervals to deter Dillon. Eventually, they start to form a wall of sorts, with only one opening exposed for a roll. Some parts of the floor away from the core will also track Dillon and project pillars where he is. When it is nearly defeated, the core's pillars will raise certain parts of the floor up and down near the central circle of pillars in a constant rhythm until it is finally beaten. The floor segments for the pillars will always flash red before they appear.
When spawning Diamongrocks, the core's pillars flash yellow and completely close off the weak-point. They will not lower until Dillon defeats the grocks.
- It is the only Grock not to have any feet (besides the Springgrock, though it maintains contact with the ground), instead floating above the ground out of reach, slowly heading to town.
- It is the only Grock to emerge from a lava lake and not a den.
- It is also the only grock to not release any ore.
- If the Ultimogrock still lives after the final opportunity, it will swallow the entire village, hinted only by a cutscene as the grock itself does not appear to be bigger than the town.