The difference there is that Kirito was aware of Sinon and knew the shot was coming.
Realistically at least, there's no reason they should have thought Death Gun knew they were there. And if he didn't know they were there, it should have been impossible for him to dodge that shot.
Of course, if Death Gun IS cheating, it either allowed him to know they were there, or it allowed him to dodge a shot he had no idea was coming.
That's actually probably the best way to deal with Death Gun.
Kirito can deflect fully automatic weapons. The Death Gun seems to just be a pistol, so Kirito should be able to EASILY charge Death Gun, deflecting any incoming shots.
Of course, that's assuming that the gun that can kill people through TV screens can't also kill someone through a light saber. But then, the intro kinda already spoils the fact that that's not the case.