Credits To wak4863 For The Vid


Mercy
Kill 5 enemies while they are on fire in Chapter 3: Fire Sale

Very early in chapter 3 you will be taken to a roof with a sniper rife, then one of your friends
will use White Phosphorus on the enemies (it looks like rain made of fire) and they'll burn.

All you need to do is shoot 5 enemies while they are burning with the sniper rifle.


Let 'em Burn
Don't kill any of the enemies that are on fire in Chapter 3: Fire Sale

Very early in chapter 3 you will be taken to a roof with a sniper rife, then one of your friends
will use White Phosphorus on the enemies (it looks like rain made of fire) and they'll burn.

All you have to do here is not fire. Just stay there on the roof until the trophy unlocks.
It should unlock 1 minute after they were on fire.

If you want to get this trophy and Mercy at the same time,
get this one first >> reload checkpoint >> get Mercy.

Remember, reloading a checkpoint will prevent you from unlocking 'Iron Man - Fire Sale'
and force you to play the whole chapter again to unlock it.