@EggsNRice
Just treat breasts, possibly one breast since it's hard to slap someone with both breasts swinging them side to side, as bowling balls or other blunt, round object.
But honestly, any amount of force necessary to crush, incapacitate or outright kill someone just by hitting them with breasts would be nigh impossible to achieve by a HUMAN, even assuming the breast in question would endure such impact. It's hard enough to kill someone just by punching them in the head, and fists are way harder and apply way more direct kinetic energy to the head than breasts would, given how malleable and soft they are.