So the various Bosses and Colossal Immortals can be farmed for powerful weapons and Arsenal parts. After beating a Boss/Colossal for the first time you can fight them again using the Rematch terminal at the base of the Base, where the Reclaimers Arsenals are located.
To craft the various Boss items you'll need the blueprint and associated Immortal Cores, possibly needing to farm multiple Bosses for the Cores.

To craft the Boss items you'll need to use the Foundry in the main terminal for the base version of said item, and the secret Foundry found in the Icecream Parlour for the upgraded versions. You'll have to look at a separate guide on how to unlock the secret Foundry.

Cybele: The base version of Cybele can be found in the large crater in the Mitera Glastra area of the Forrest map. You'll more then likely come across this naturally while exploring the map for the first time.





Cybele RT a: The first variation of Cybele can be found just outside the lower fast travel point in the Everes Petra Ruins Reserve location of the Desert map. You'll need to progress in the story somewhat before it'll properly spawn, however.





Cybele RT b: The second and so far last variation of Cybele can be found in the northern part of the Martel Barrens in the Mountain map. You'll need to progress quite far in the story for Cybele b to spawn. For me I had to complete the entire story before I could get it to spawn a single time for me, your luck might vary.

Dreadnought: The base version of Dreadnought can be found in the massive Sandstorm at the center of the Magosthine Highlands in the Desert map. You'll need to progress the story until the Highlands opens up to you, as the Highlands aren't available out the gate.




Dreadnought RT a: The first variation of Dreadnought can be found in the Riza Stoma Barrens in the Desert map. You'll need to progress the story to the same point where you gain access to base Dreadnought to fight Dread a.




Dreadnought RT b: The second and so far latest variation of Dreadnought can be found in the wide open are of the Lytron Basin in the Mountain map. Strangely enough, this is the same location Rebellion b can spawn, so you might have to reload the map a few times to get Dread b to spawn.

Eclipse: The big orb from DxM 1 returns and can be found just next to where Dreadnought and Rebellion b can be found on a raised area. You'll need to beat the main story before Eclipse will spawn.
As far as I am aware, there isn't an Eclipse RT a in the game, yet. There's going to be more bosses added in future updates so we may see *golden* orb with that update.

Grauam: The base version of Grausam is located just outside the fast travel at the Stefanos Barrens area of the Desert map. You already know this as he's the first Colossal Immortal you fight as part of the main story.





Graunsam RT a: The first variation of Grausam can be found nearby the center fast travel of the Hados Plains area of the Forrest map, and will likely be the first variation you find as you explore the various maps.





Grausam RT b: The second and so far latest variation of Grausam can be found right next to the Oros Lyssa Gorge fast travel of the Mountain map. And will more likely be the first boss you encounter upon entering the Mountains for the first time.

Gunfort, RT a, RT b: All the versions of Gunfort can be found right behind the Aristos Hill fast travel of the Desert map. Gunfort will tier up throughout your story progress, so early in the game it'll be base Gunfort that spawns, then Gunfort a, then Gunfort b.

Lapis Lacrima: The base version of Lapis Lacrima can be found at the center area of Adelphos Dunes in the Desert map. You'll first come across this boss in the main story while doing a quest for Forge. However after fighting Lapis for the first time it hasn't spawned in the world again for me, so you'll need to farm it using the Rematch terminal in the Base.


Lapis Lacrima RT a: the first variation of Lapis Lacrima can be found in the Makarios Weapons Foundry area of the Desert map. Strangely, you'll likely have encountered Lapis b first as it's located in the Forrest map.





Lapis Lacrima RT b: the second and so far latest variation of Lapis Lacrima can be found right at the top of the Forrest map in the Anesis Forrest area, in the great big water area that screams "boss arena".

Rebellion: the base version of Rebellion can be found at the bottom left of the Forrest map in the Philia Plains area, in an arena that looks like the entrance to some kind of ancient temple, shame we can't explore said temple.





Rebellion RT a: The first variation of Rebellion can be found in the southern area of the Martel Barrens area of the Mountain Area, just to the left of the fast travel point.






Rebellion RT b: The second and so far latest variation of Rebellion can be found in the wide open are of Lytron Basin in the Mountain Map. However this is the same spawn location of Dreadnought RT b, so you might have to reload the map a few times to get Rebellion b to spawn.