From the Game's Introduction:
Achilles' Heel is a game for two players - one will play a super-powered hero, the other their nemesis. Choose who will be the superhero and who will be the supervillain, and quickly choose names for your new personas - Captain Fantastic and Doctor Midnight, for example. Find ten cards or slips of paper to write on, and fifteen pennies or tokens of some kind. The hero will quickly write down ten things on the slips of paper, numbering them from one to ten. (Obviously, the players may collaborate on the ten things if the hero is having difficulty coming up with them.) The things may be objects, actions, places, emotions, anything. Two of these things will be the only weaknesses that the hero possesses - otherwise, the superhero is completely and totally invulnerable.
Before starting, each player draws four slips randomly from the pot, with the remaining two representing the hero's vulnerabilities. Thus each player has non-overlapping partial knowledge of the hero's invulnerabilities. The slips are returned to the pot.
Play consists of five "episodes" followed by a final showdown. In each episode the villain presents a situation involving two of the aspects drawn randomly. They place a number of tokens equal to the episode number, split between the two aspects as they see fit.
The hero selects one of the aspects to resolve. If they are invulnerable to this, they take those tokens and the villain gets the rest. If the chosen aspect is a weakness of the hero, the villain takes all of the tokens.
In the final showdown, the character of the player with the most tokens will be victorious.