Alberto yes, it is the other way around
Your kryptonite is your Achilles' heel, that is to say your least fav technique or the technique that usually fails you; your weakness.
Kryptonite is Superman's Achilles' heel, that's it.
I admit I didn't know this as well (the kryptonite business) but google gave me a hand
Other than that, thank you for your comment anyway
Grazie!