How about "Ultimate Weapon". I am thinking here of things like Greek Fire, Archimedes claw, The supposed Archimedes death ray, things like that. Civic, cost 240. 10 Credits in Civic, 10 credits in science. Advanced Military gives 20 credits towards it. Allows you to automatically win any 1 battle losing no units.

Also, how about a new science card "Alchemy". Science 90, 10 science credits and 5 craft credit. Gives 20 credits to Medicine.

Would your Ultimate Weapon winning one battle include sacking a city with one token?

P.S. I'm not saying that's a bad thing. It's no more powerful than Monotheism or Politics so it's not really unbalancing as long as it's limited to one battle.

Well no because technically one unit against a city is not a battle. Good question though.

I'm thinking a name such as Elite Troops or Palace Guard may be preferable to Ultimate Weapon. Troop quality is usually more important then the wepons they use, and it has a wider application then the Greek weapons you mention such as the Preatorians, Alexanders' Companions, and the Persian Immortals for example.
It may also be fun to tweak Alchemy a bit to have an ability to count as an extra Iron card in a set or something similar.