At the end of episode 3, you will have a boss fight with Mutant Neil........just lure him into one of those yellow crates, have him try to hit you, evade to the right, and keep luring him into the fire until he says, "I'm burning up!" He will then run to one of the pillars to cool off, unload on him, then repeat the process until he is dead..........
Here is where the achievement comes into play
......you will have a cutscene, where Neil breaks into the elevator, at the end of the cutscene, you will encounter an
prompt......DON"T DO IT!!!!!! Change to Moira by pressing
instead, crawl towards the gun, THEN you can use the
prompt, then the achievement should pop up!
I understand this may be obvious to most people, but I'm sure there are a couple people who didn't know they can switch to Moira........hope this helps someone!!