Warp
12 Achievements
200
12-15h
Crossfire
Two soldiers shot each other trying to kill you.
15
How to unlock the Crossfire achievement in Warp - Definitive Guide
Edit: You can do it on the echo challenge which isn't too far after this point in the game but is a good place to do it if you can't get it earlier.
Thanks mysticweirdo for reminding me to put this in.
For this trophy you will need to get two soldiers to kill each other simultaneously. There are a few instances when you can do this. You will have to create an echo to walk between the two, and have them shoot each other. If they kill each other, the trophy will pop.
This can be done in either story or challenge modes. I suggest challenge mode, as you can just keep retrying and not have to rely on getting back to where you were if you died. The challenge I suggest is found early on in the game and is the second challenge you come across called "Frag". You must kill all enemies in a time limit. When it starts, go to the bottom left area and you will see 4 enemies all close to each other. Warp around and get them to shoot each other at the same time.
Alternate Method: What might be easier for some, is to just wait until you have an ability that lets you walk around in a type of "ghost" mode. It is called Echo. Run the invisible alien in-between two enemies and you should be safe as they fire at one another.