When Castiel tells Dean that he has found Crowley, Dean replies, "Okay, Huggy Bear. Just don't lose him." He is referring to the confidential informant from the 1970's TV cop drama, "Starsky and Hutch."
Lucifer says that there are only five things in all of creation that the Colt can't kill, as of season 12 four of them are known. It can't kill: God, The Darkness (God's sister who went by the name Amara), Death or Archangels. The fifth thing it can't kill has yet to be encountered.
The trivia items below may give away important plot points.
Jo Harvell dies in this episode the same way that her father was described to have died in Supernatural Season 2, Ep 14, "Born Under a Bad Sign". Sam (while possessed by a demon) recounts to her how her father had died while out on a hunt with his father (John). John used him as "bait" and he "died trying to hold in his insides" after being attacked by a monster. This is the same way Jo dies before the bomb is activated.
After Lucifer is shot with the Colt he says that's the Colt can't kill five things in all of creation, it would stand to reason he is referring to these five things as being, himself, the archangels Gabriel, Raphael, Michael and lastly God.