Me and my friend lucas dealt a million damage and still didnt get the badge....
The 'One in a million' badge only goes to the person who hit, not the team who supported them. It must be dealt in one hit.
The badge can be found under 'Miscellaneous Badges' that has a big yellow star right at the top of your badges. It will show in the 5th slot down on the first page.
If you made the hit, it was 1,000,000 in one hit and the badge is not showing in your book then it would be good if you contact KI to let them know it is bugged.
I hope you find the badge is there when you look for it
