The Scarmiglione is a lizard like creature with "shield" and "sword" like appendages. It appear near the end of the game (scripted and unavoidable), as well as later levels in the Raid Mode.
I was able to get this upon finding the
. I had a fairly powerful Sniper and simply popped a couple of headshots on the creature. You can trick it into protecting its legs/abdomen by shooting it in those areas with a weaker weapon, and switching to headshots once it lowers the shield.
Alternatively, if you want to waste (or reload a save) it, you can just blast one with a rocket launcher (I believe you get one shortly before the
times section.