Not sure why this is considered unobtainable - maybe content added over the past year did something? You can still do this quite simply in Brawl game mode where players have three lives. Minimum 3 players needed. Once one players loses all 3 lives, take their lifeless body by pressing and holding circle button, point towards another player with only one life/knockout remaining and then throw. The trophy pops once you finish off the final life from another player with the thrown dead player. There are hearts above each players head indicating how many lives/knockouts remain for them.