First and foremost, pick Brock Lesnar.
To execute "German Suplex 4", press A behind a stunned opponent.
To execute "Back Suplex 2", press Up on the left stick and A behind a stunned opponent.
Best way to grapple behind a stunned opponent is to flick the right stick left or right to turn him around. Any other grapple suplex from behind won't work here. If you can't turn him around, you know he's not stunned.
You can alternate between both suplexes and the achievement will pop as long as you do 10.