The ghost is actually quite vulnerable if you think about it, he sits in your pack and hides unless you need him. The ghost shells only protect like 80% of the ghost so if the dead Gaurdians happened to have their ghost out, it could have been shot. Now assuming that that wasn't the direct cause of death for the Gaurdian, considering that we were long dead when they found us, then the Gaurdian could have fought until he died but couldn't revive. Or The ghost was shot while trying to revive the guardian. Now, I know what your thinking: [spoiler]but mighty how did the ghost get into the temple of crota during that mission where you first spot the stranger on the moon? [/spoiler] I'm getting there, be quiet. The ghost was being interrogated by a wizard inside of the temple and died from the interrgation, giving further evidence that ghosts have little to no armor. (Thanks TKS597) So... In conclusion: The ghost died first.