Sunset Overdrive

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.

Heres a place to grind out a few kills. In the mission to rescue Bryllcream you have to open the truck he is in. At the point where you are on the roof and you try to open the truck, the truck moves down the slope and you are attacked by OD. Just stay here and keep grinding the kills. You can also use this to level up your weapons as well. Ammo and health spawn frequently. I have the Amp that bursts flames as you grind. As you can see I've got a x999 combo. If you get knocked off the rail try to land back on it again. If you fall down you will break your combo, as I did at the end of this video clip.