Well, he is does have the keywords "Chaos" and "Daemon", so anything that does extra damage to Daemons might be a good idea. Now I have not played AOS more than once or twice to try it, but I would guess that we are one of the armies better suited to taking him out.
The units I can think of, that does good damage to daemons are:
Chameleons skinks (I think these would be a good choice, as you can try to deal with him quickly and can dish out some real pain, granted he does have 20 wounds, but I think you can have a lot of chameloen skinks for his price. You probably would want to split the units up, so he can only wipe out 1 unit at a time).
Bastiladon, with the solar engine, as that can potentially deal up to 36 wounds (this won't really happen).
Oldblood on Carnosaur, might do something, iff (this is not a typo) you get him to attack first, and is a bit lucky and gets him pinned down (remember he is a monster). You might also do a little bit of damage with his shooting attack, that does 2 damage to Daemons.
Kroak, he might be able to more or less just kill Kroak outright (more or less), but Kroak is sort of immune to his instant kill. Kroak also can give some protection to your other options for fighting him (Arcaheon only has -1 rend so mystic shield might do something) Kroak also has his spell, that he can cast 3 times (you would probably want an Astrolith bearer) that does D6 mortal wounds against Chaos Daemons, which might also be the rest of his army, so you might be able to focus more on Archaon, as his other units, might just be obliterated by Kroak. Granted Archaon does have an ability that ignores mortal wounds on a 5 or 6, but Kroak can deal a lot of mortal wounds to him, that it might just be enough, but his ability so he might just ignore the spells, id nasty, but then it is good that Kroak can cast the spell more than once, and probably won't have to fight him alone, with the other options, that you can use to slow him down, and wound him.
(Make sure that you don't have a ton of heroes, or that the heroes are hard to get to, as he can heal wounds if the kills them)
Now, go and kill that foul creature, and fulfill the old one's plans (even in space), like we do in the other dimension of the universe, where the old world is still intact.