Mastodawn

requirements: a "sieker" gun

first, get into the library room through the window and knock down the servant lady there, her actual position is watched by security, but 47 can toss a coin or gold bar to attract her to the wooden chest-that-can-store-two-bodies, then knock her out and hide the body
go back through the window, enter the mansion the normal way, get up to the second floor to the corridor over the sitting room where the announcement is about to be made
find a good place where it's possible to do so, then fire an emetic dart gun at the target without being spotted
run back out of the mansion and through the window to the library room, then to the bathroom nearby, right as the target enters it
DROWN
leg it out of the bathroom window and to the mission exit, the body will be found, but since 47 did an accident kill, it won't break the silent ass