Unlock Scarstu Debris Field planet

Go all the way to the far end of the map and enter the bar, at the end of the bar you will see an arena and a terminal which you can interact with

Interact with the terminal, and press the first option "Bronze"

You will see a bunch of challenges the one you want is at the bottom called "A Grunthor Named Sue", if this is your first time then the reward you will receive won't be Raritanium but it will instead be a golden bolt, after completing it for the first time go back, and you will see that now the prize you will earn is Raritanium instead, it says that you will only earn 2 Raritanium when defeating the Undead Grunthor but that's not true, every time you complete it you earn 5 Raritanium 3 from defeating the boss, and 2 for completing the arena

repeat as many times as you want

If you have the Lombax praetorian armor set you earn 20% more Raritanium