It's the middle of the afternoon that's the hardest. Here - have a lovely frag video to make things easier on you. I don't know why defending like this didn't occur to me when I was playing CS: Source. In hindsight, it seems so simple.
For those who never warred on the mean streets of Counter-Strike, this is de_dust2, the series' most popular map by far. And what you've just seen is only half fluke - he would have practiced strafing and shooting at that exact spot hundreds of times, just to maybe pop a sniper at the other end.
Even if they have bought snipers, these enemies aren't expecting a kill through that narrow gap - the best they can conservatively rely on is counting how many black shapes they see cross it, telling them which side of the map has how many defenders.
But having them all line up like that for a long range pistol shot? That's one in a million, baby.