In the first level. Act 1 Chapter 4, i think. There's a very large boss who you must shot the core to kill. In this battle it is almost 100% likely that one of your team mates will go down, just walk up to them and press x when they do.
If this fails, Soon after you have to save troops from some strange trap things, after doing this they are downed, so you can get it there if not before.