What do we know about Zeno:
— He has overpowered abilities; a single cast can disable our entire group and obliterate them. Simple attacks also take characters out of the battle with one hit.
— He has 25 initiative, meaning his next turn will wipe out our group. I don't think even Artery's defense will help here. If we survive, we might be left without money, weapons, and other joys of life. In short, things will be very bad if we don't defeat him in one round.
— He likely has a lot of evasion and accuracy – Anders' hit isn't indicative, as he has a huge amount of accuracy + an ability to reduce evasion.
— He has 1000 HP (640 remaining), no resistance to physical or lunar damage, and fire resistance won't protect against Air. There's also no immunity to critical hits.
What conclusions can we draw from all this? The group needs to deal 640 damage to him before his turn begins; otherwise, it will be very, very painful. And that's assuming he doesn't have any nasty passives, like corrosion, that break weapons on hit. Technically, if there isn't an overwhelming amount of evasion and no defensive surprises, Air alone might be enough. However, given Revan's reluctance to let us fight this guy, I can conclude that there are indeed some surprises in store. Something like that.