Inside Fort City, during your pursuit of the Bahram Scientist Lloyd, you will be forced to continue down a network of shafts with moving "pistons" or columns that close in to block off further passage down that tunnel and into the next room. Simply wait near piston or moving death column (whatever you wanna call em) and wait for an enemy to approach you back away to prevent your own instant death and bleepbloop-alternatively you can Geyser them to immoblize them straight in the path of the pistons, move away and PROFIT!!
To put it simply: you get an achievement just for taking advantage of herp derp AI piloted Mechs killing themselves!