Our base seems nearly impenetrable, but our lack of a missile defense system is our Achilles Heel.

Maria is an excellent athlete, but after an injury she suffered two years back, her right knee has become her Achilles Heel.

I have excellent skills in almost every academic subject, but mathematics is my Achilles Heel, and I have a D in it.

A knight wears a full suit of plate armor that is almost impenetrable, but the gaps in between those plates are his Achilles Heel, being vulnerable to attack.

Even the strongest, most powerful individual has an Achilles Heel in the form of their loved ones, which will often force them to stand down if they are in danger.