I had absolutely no plans for the details of the binding spell. Probably just something that is used on demons in general, so some Angelic symbols could certainly be part of it. And it not working on Steph would be a great twist. If they get so far in actually using, but even if Mike falters for some reason, Stuart Greyhill will attempt it or finish it. (Just don't kill him, I need him for the time-skip after they leave. Mike is going to stay with him for a bit to learn more about magic. He's not a bad guy, I swear)