Doom 2 – Why Shooting at the Wall Kills Enemies

It turns out that you have to use the door to the tower—the wood-paneled wall to the left of the screenshot. (I didn’t even notice that it doesn’t close all the way because the top is not visible.)

You have to open the door, stand on it, then wait for it rise, taking you up along with it, and then run across the gap.

The chance of "stunning" an enemy is called "Pain Chance". Each monster has a certain chance that they'll enter a "pain state", in which they flinch and stop moving and attacking, each time they're damaged. This chance is fixed, regardless of how much damage is inflicted, and depends only on the type of monster that was hit. For example, Lost Souls always flinch; pinkies flinch around 70% of the time; and the Cyberdemon flinches about 8% of the time.

Each projectile fired by a weapon triggers a separate roll for pain chance, so rapid-fire weapons (like the chaingun, plasma gun, and chainsaw) have the most chance to inflict pain state. The shotguns also roll separately for each pellet, so they're effective as well. Weapons firing a single projectile, like the pistol, rocket launcher, and fists, only roll once, so a pain state is much less likely. (The BFG does damage on impact, plus additional damage with "tracers" that are cast from the player's position in the direction the BFG was fired. Enemies in the path of these tracers tend to get hit multiple times and frequently flinch as well.)

Sources: http://doom.wikia.com/wiki/Pain_state and http://doom.wikia.com/wiki/BFG9000