Achilles' heel

there is nothing wrong with this phrase--it is in fact useful for both sexes, popular, and irreplaceable by anything nearly as apt--but be aware of how many such expressions are male-based. Balance their use with female-based expressions, creative expressions of your own, or sex-neutral alternatives: vulnerable point/spot, vulnerability, only weakness, chink in one's armor, where the shoe pinches. See also sex-linked expressions.