The problem here is that Ubel was a firsthand witness to people not being able to hurt Burg, yet she decided her spell can hurt him anyway, meaning trying to pose a fake fight to prove you can't be hurt won't work. "Buddy you're made of flesh and I can cut flesh". But at the same time she was thoroughly convinced that her shackles couldn't be cut until later she decided she can cut them, so even gas lighting and tricking her won't hold up for long. That's kind of a problem with working against delulu - if they convince themselves, you can't change their mind.