The physical feat that Mary Maloney performs is that of killing her husband Patrick by hitting him over the head with a frozen leg of lamb, as described in the following quote.
Mary Maloney simply walked up behind him and without any pause she swung the big frozen leg of lamb high in the air and brought it down as hard as she could on the back of his head.
She might just as well have hit him with a steel club.
This comparison of the leg of lamb to a 'steel club', and the fact that it kills Patrick instantly, shows what a formidable weapon it is. It takes some strength for Mary to wield it, but the sheer shock of being told by Patrick that he's leaving her seems to jolt her into this extraordinary action. As can be seen from the above quote, the murder is related very matter-of-factly; it is something that Mary does without stopping to think at all.
The choice of weapon of course is very unusual, and lends the whole story an air of black comedy, a style that Dahl was famed for. This comic grotesque strain is especially evident at the end, when Mary succeeds in getting the detectives investigating Patrick's murder to eat the leg of lamb - thereby getting rid of the most incriminating piece of evidence.