(Many years and SSRS incarnations later .. it refuses to die in spite of many attempts by Microsoft to murder it. It's now known as "Power BI Paginated Reports"!)
Create a fake report using the "Table or Matrix Wizard", use a simple SQL query, preferably no parameters, but with the same (numerous) fields as your Target report.
Select "Available Fields" all together and drag them into the Sigma Values box.
Voila, a Tablix is created with all the fields.
Copy Paste the Tablix into your Target report. Change the Dataset for the Tablix to the target dataset. Apply any formatting to the header and row.