To try to construct a general reply to the points raised.
It was on PC. Regarding hax, the QME was not the host (Siren was hosting) and he joined when the game was already in progress so we never got to see his loadout. The enemies were staggering even from a bodyshot - since it was a PPR I could see where he was shooting, obviously. whatever he was doing wasn't terribly effective, since he was outscored by a QFI with a Paladin (me), and was 3rd on the scoreboard. He never threw any arc nades that I could see, and the game wasn't bugged for me anyway. the ammo wasn't incendiary or disruptor because those produce visible effects that I would have recognised. Explosive staggers after a delay, not instantaneously. Phasic doesn't stagger either, does it?
I was very conscious of the effect, because of course he staggered everything that I had lined up for a headshot. When I saw the effect I called it out to Siren so she came over to check it out and saw it too.
So, what am I missing?