It shows you in the arcade itself. Start in the normal headquarter, and BEFORE you click on match up #1. Read the "INTRO". A small box there will tell you exactly which clans you will encounter.
(Side note: I still really dislike the decision to mix clans in individual headquarters. I versed a Sakhrom/Vortex (/wGuru Cr) in match-up 10. Nothing to do with Freaks at all in the final (non hardcore) fight.
Correct me if I'm wrong, but what I think Graham is asking for is basically a list of the decks that each stage of each HQ uses, in an effort to try to counter all the possibilities in each stage.
I don't think that anyone has put the time in to make a list like the one that you are requesting. I would love to help you make this list, but I unfortunately cannot even beat stage 8 of the first HQ in Season 3 due to my lack of a good deck.
That's what i'm asking for @NayrSlayer I know the clans in every HQ but it battling against Frozn or Vortex/Nightmare is a completely different thing...
I'll do my best do create that list... If anyone wants to help just posted here or PM me.