If I have an Elesh Norn, Grand Cenobite equipped with a Blade of Selves and I attack with her in a 4 player game (me and 3 opponents) do my opponents' creatures get -6/-6 long enough for state based effects to check and kill larger things?

Or do I need to turn to Rules Lawyer or Mirror Gallery to get the kind of oppressive shenanigannery I want?

all state-based abilities are taken simultaneously, so yes, the game will simultaneously kill your duplicate Cenobites and your opponent's 6-or-less-toughness creatures.