Near the beginning of
Chapter 3: Fire Sale, you will be lead to a roof with a sniper rifle, looking down at the Koreans in a big parking lot. After a few moments of watching,
"White Phosphorous" will be shot, which basically makes raining fire. Your female companion will tell you to put them out of their misery, instead of shooting, just wait 10-20 seconds, and the trophy shall pop.
Now if you want to aim for the
The Mercy trophy in Homefront worth 36 pointsKill 5 enemies while they are on fire in Chapter 3: Fire Sale , pause the game, then
Restart Checkpoint, and when she tells you to put them out of their misery, go ahead and shoot as many as you can (5+). If you fail to kill enough and all the Koreans just died, restart the checkpoint again, as it will not reset your kill count.