This achievement requires you get earn any one of the 65 different onyx medals. The easiest and fastest medal by far is the one called "Doorman" it requires you to manipulate 2000 objects in the campaign.You earn credit towards this medal everytime you press a button, pull a lever or open a door etc.
Load up Act I Chapter I, after the cutscene with Jace you will climb up a staircase and open a door, right in front of you will be a piano. Walk up to it and press X to "play" it. Mash your X button for about 1.5 hours and you will have the easiest and fastest onyx medal in the game!
You should also keep in mind that you will earn a onyx medal for completing the game on insane, if you plan a playthrough on that difficulty you won't need to worry about this achievement.
For the full list of onyx medals, check out my Seriously 3.0 guide: