A Secondary Dimension gives this report more flexibility once it's exported, reducing the number of different reports you need to process.

Choose a Secondary Dimension that adds context to the Primary Dimension. In this case, "Campaign" is a very useful way to split Source/Medium.

Search for your chosen secondary dimension. Here, I type "campaign".

Click the dimension when it appears. Here, I click "Campaign".

You want to capture as much data as possible in a single report. Try setting a date range back to when you first opened your shop or installed GA! Only make it shorter if your report ends up with more than 5000 rows.

Click the arrow next to the date range, top-right of the report.

Click the left-side date field ("Starting date")

Use your keyboard to change the year in the field and hit "Enter".

If you have trouble directly typing into the date field, you can use the calendar "picker" to select the new date instead. Directly typing should be easier if we're going back many years!

Check how many rows the report now contains. If it's less than 5000, you can export them all in one go!

If you see the number "5000" here or more than "5000", adjust your date range to include less time. You'll need to follow this process multiple times to export the report in "chunks" of 5000 or fewer rows.

Change the report to show "5000" rows on one page.

When prompted, choose a safe place on your computer to save the CSV file.

You can open CSV files in Excel, Libre Office, Google Sheets or any spreadsheet tool. CSV is a flexible format that can hold a LOT of data - more than an XLSX file - which is why I suggest choosing that format for your initial export.

analytics.google.com

This guide can be used on built-in and custom Universal Analytics reports.

Export a "Universal Analytics" Google Analytics Report to CSV