Like most people, Achilles had two heels. But his were of different interest to posterity. The one that gets all the attention is the one his mother Thetis held him by while dipping him (as a baby) into the River Styx; the other was the one that (along with the rest of his exterior) touched the waters during his immersion and thereby became invulnerable. In one sense, then, it is hard to come up with a more suitable and exact antonym for "Achilles' [vulnerable] heel" than "Achilles' other heel."
Regrettably, there appears to be no consensus about whether the vulnerable heel was the right one or the left one. This explains why, in the illustrations accompanying the Wikipedia article on Achilles' heel, the painting by Rubens shows him being held by the left heel, while the statue of Achilles dying shows him grasping the shaft of a spear projecting from his right heel. If Homer had been more specific, we might now be able to use the clear antonyms "Achilles' left heel" and "Achilles' right heel."
Instead we have to find the figurative opposite of "Achilles' heel" elsewhere—perhaps in the form of the invulnerable part of something otherwise all too vulnerable, such as "France's Maginot Line." But that doesn't make a good pairing in the OP's original wording:
I will tell of their Achilles' heel, and their Maginot Line.
Nor for that matter does the example of complete physical invulnerability that the Good Witch of the North's kiss gives Dorothy Gale in The Wizard of Oz fit the bill, despite the fact that being completely invulnerable certainly constitutes a fundamental contrary to being vulnerable in only one place. It just doesn't work as part of a pairing of metaphors:
I will tell of their Achilles' heel, and their Good Witch of the North–kissed forehead.
The problem, as my two attempts to complete the pairing that the OP seeks demonstrate, is that bringing together a metaphor from a Greek myth and a metaphor from any other specific but unrelated source is like hitching a mule and an ox to a plow: They don't work well together. I recommend minimizing the contrast between "Achilles' heel" and whatever you pair it with by making the other element as nonspecific and nonallusive as possible. For example:
I will tell of their greatest strength and their Achilles' heel.