Did you try - Analytics/Canpaign Reports/marketo Campaign Email Performance?
Subject Line Testing can be helpful too.
Hi, gang! There are two ways to make this work:
The second method works across multiple programs if you want to keep a single report for a series of programs in a Campaign Folder:
You should see correct reporting results this way.
ADDENDUM: It turns out the second method may not show correct results. We have tried it in an email program series, but the reporting is way off, showing ten times the number of actual outbound sends. Our program set may be unusual, as we had different numbers of champion/challengers (A/B or A/B/C),. So check your results, and probably stick with the first method, particularly if your email programs use different numbers of subject line tests.