If and when TSHTF, get the best available cover, identify the threat, and if necessary shoot, move, and communicate. In the event that after shooting, movement and communication are not possible, become and remain the smallest target possible. That usually means "becoming one with the ground," facing the threat and watching your "six."
As Sun-tzu wrote in The Art of War:
" Thus, one who excels at warfare first establishes himself in a position where he cannot be defeated while not losing [any oportunity] to defeat the enemy."
(Sun-tzu, The Art of War, Ralph D. & Mei-chin Sawyer transl. 1994, at p. 184.)