I don't quite get this game yet.
I am playing Empire and already at turn 130 or so. Archaeon just showed up and I only have Reikland province and another small neighbouring settlement. I only have enough money to upkeep two full army stacks which are currently split across three armies that are perpetually stationed in my settlements. If I leave my settlements unguarded and go out to try to conquer shit, the default garrisons will get smashed by those Greenskins or Skaeland assholes which sets me back a lot. So what do I do now?
Are you in alliances with everyone else? (The Shield of Civilization trait is added to "civilized" factions during Archaeon's appearance and makes alliances easy, FYI) If not, do so immediately; establish trade routes, confirm alliances, and direct your allies' armies (from the diplomacy screen) to attack Chaos' invading stacks--and make sure the command sticks. It is critical that you start hitting Chaos with a united front ASAP. The extra trade money should help you muster a larger army.
Depending on how strong everyone else is, they might be able to weaken Chaos enough for you to drive them back. But it is critical that you wipe out both Chaos and their Warherds in one go, or else Archaeon & co. will return after a brief absence to wreak even more havok. And this time, everyone will be even less prepared.
Now, between Norsca and the Greenskins, if you can redirect any allies too far away to realistically hit Chaos into their armies, you can (hopefully) keep them busy in the meantime. As much as you might want to, you cannot realistically engage them without giving Chaos time to build momentum, which is worse.
Like, you're in a tight spot, but you'll almost certainly have to intervene if you want to wipe Chaos out; sacrifices have to be made. And once those alliances fade, you're going to be in an even tougher spot; Norsca will quickly offer peace, but everyone else will be eyeing the remaining lands for themselves. Plan out an expansion, because the post-Chaos peace will
not last.
Empire is really challenging as far as the Chaos invasion is concerned. They're both at the brunt of the attack, and the key to defeating them.