Mighty chudan set is a monster set.

These 2 piece monster sets are not tradeable. None of them. They have to be earned by completing daily pledges (handed out at the undaunted quarter) check here: https://elderscrollsonline.wiki.fextralife.com/Undaunted
and here:
https://elderscrollsonline.wiki.fextralife.com/Undaunted+Pledges


In general you may get any shoulder piece by completing any pledge. This is possible because completing a pledge will grant you a minimum of one key (normal difficulty). If you like to trade that key for a chudan shoulder you will have to try your luck with the Urgalarg Chief-bane's chest which can be found at the undaunted enclave.

If you like to get the mighty chudan helmet/mask/visage you will have to complete the dungeon "Ruins of Mazzatum" on veteran difficulty. Quality and trait of dropping monster piece is again purely due to RNG.

As a sidenote: every weekend the merchant named Adhazabi Aba-daro The Golden (https://elderscrollsonline.wiki.fextralife.com/Adhazabi+Aba-daro+The+Golden can be found in cyrodil. She offers 2 monster set pieces per week, both are randomly choosen but usually we find either helmet or shoulder pieces in her stock. She sells them for 100k gold or 200k AP per piece. All monster pieces offered come in all 3 armour categories (light/medium/heavy) and 2 traits: infused (sold for gold) and impenetredable (sold for AP). Mighty chudan helm as well as shoulders have been sold in the past already (However, no-one knows when they will be back).

Hope this helps, enjoy ... and yes you should get 3 other ppl to do pledges with you (if you are just beginning)

You have to get it from Urgalarg Chief-bane's chest. Its RNG based, so theoretically, you should eventually get a pair