I'm not sure if anyone else has experienced this but I have "killed" a harpy but it didn't die. It no longer had a health bar or name, but was still flying around and could shoot and damage me, and every time I shot it back be it my primary, special, or heavy weapon, Melee or super (golden gun, fist of havoc, ect...) it will say immune every time I hit it. And it cost me the lvl 28 weekly heroic strike against the nexus. (Solo) I couldn't kill it while I was behind cover from nexus and nexus was inches from death, so I had to move to different cover and ended up dying. I was unbelievably pissed. Don't get me wrong, I love this game and think it was $60 well spent and try to get the bounties and daily heroics done everyday if I have the time, but their are minor glitches and flaws about this game that are very annoying. This being one of them. Keep up the good work and keep those challenges coming! Hopefully without the immortal enemies lol.