I hope I am not missing some major mechanics with this, and sounding stupid, but would it help using auto attacks in combination? I don't know if there is a certain threshold needed to interrupt with them, but it's worth a shot. Unfortunately I can't say much of anything because I haven't gotten to that point in the story at all, but it figures I'm going to be completing it with either SCH or AST :P
Edit: Major derp, I don't know why I was thinking that xD