This one is very straightforward - simply kill 5,000 OD. You will get this by playing the game and going for other achievements.
If you don't want to wait, a mission that's good for farming OD kills is called "Getting The Band Back Together" when you need to help your drummer. Don't follow the instructions but keep bouncing on a drum. The OD constantly swarm you so you can just bounce and then press
to slam to the ground or shoot from any gun.
You can replay this mission from the Log in the pause menu. Note that the mission happens later in the game when you've met Las Catrinas.