Cockatrice can't be fought yet, that encounter will mark the end of Chapter 1. Megumin's and Rem's panties are the only ones you can Steal (aside from Chris's, but those are plot-related)
Seriously this game is top tier man, I'm glad to see that its been working for you so far. It'd be nuts to be able steal mostly every girls' panties and have them as collectibles (maybe related to an achievement), and even get optional scenes related to that.
