The way you obtain it is complete all the spartan company challenges, making it the only set in H5 that you have to work to unlock rather than just getting lucky with a req pack.And do you have to be in a company to get it or can you do without it?
Are you asking why it's so heavily requested for Infinite? If so, probably because it was one of the good looking new sets in H5. When you have so much garbage, the good ones tend to stand out more. I personally think it looks really cool too, thought I like my Buck helmet and Jun armour combo more.
Achilles is earned through Spartan Companies. You must be in one to unlock it.
You can’t join a company that already has it unlocked.
You have to be a member of a company when they unlock it to get the armour. Same applies for the helmet unlock too.
There are a few companies close to getting it that are looking for members. There are also a lot of companies who are trying to get it but probably won’t due to how much effort it takes to unlock.