Go to the trench with the 2 mounted guns. Walk towards the bunker from there.

At the entrance, look at the small wall on your right that juts out from the trench and becomes the bunker wall (sandbags).
Face the corner and do small jumps to glitch into it (it takes a few tries and is done the same way as the patched rust pipe glitch was).

When you are in the wall, other cannot see you, but you can shoot out from there.