I cannot reproduce this yet at this specific location. However at other locations it can be reproduced like this:
1. Move character in contact with rocks.
2. Walk along or over the rocks, especially focusing on angles or crevices in the rock face. You will find some locations where coming into the rocks and pushing against them (as to climb up them) causes the your character to vibrate back and forth. Continue to move and push against these rocks until the rocks kill yourself, your character passes into the rocks.
3. Move to advantageous positions on the border of the rocks, on the inside, and test for locations that allow you to fire bullets from within the rocks, to the outside, by firing and watching impacts.
It is easier to reproduce this by falling into crevices and pressing against the walls. Crevices that are not large enough for your character to stand, do not know how to interact with the player and this glitching out and phasing into the rocks is far easier in this way.