: As for the Mantis in the final fight, I found this...
: https://www.youtube.com/watch?v=YoxmyXQH0g0

In case you didn't know, there's a easter egg on that level, before entering the building.
I found it on a ledge next to a Covie Intel.

I may be wrong on this, but every time I shot the item, no sparks appeared. But a grunt will show up in a Ultra Ghost. Very useful in the next fight. I'm assuming the ghost is the easter egg. I've taken out every enemy in the level and the Ghost only shows up after I shoot this thing. There doesn't seem to be a limit on them either. I was playing alone and got six of them to show up. Only one at a time though. The Grunt is a terrible shot. Simply board the ghost to acquire it.