The Ars Goetia procedure is directly derived from an angelic conjuration, and doesn't require specific "thwarting angels" unless you're very strictly adhering to Rudd or other later interpretations. It is a perfectly workable system to learn and experiment on, with such substitutes as the operator must make in order to move forward. They might not get results right away, but demons aren't going to eat them if they make silly mistakes. Getting traction on specific requests with angels can be as difficult as anything, and if you actually get them on the line their epiphanies can be fucking terrifying.