It depends on how you are trying to "flatten" the nested XML into CSV fields, you can try UpdateRecord, JoltTransformRecord, etc. Each record-based processor does different transformations, but in the end they can all convert from XML to CSV, you just have to tell the right processor what to do. If you provide more information here (sample input and output, e.g.) I can try to help get you going.
@Matt Burgess I am also thinking to convert deeply nested xml to csv, and thinking of using either of ConvertRecord, UpdateRecord or JoltTransformRecord. What are the difference between UpdateRecord and JoltTransformRecord? Which one should be suitable?