After completing the game plus horde and playing online I had around 3,000 kills. For much faster method to Seriously 2.0 you can rinse and repeat the method in Act 1, Chapter 4 (Tip of the Spear, The Big Push) or you can keep loading the chapter in the video below where you ride the Brumak Act 5, Chapter 5 (Aftermath, Closure) Just launch your missiles at the two squads approaching you then reload the checkpoint and repeat.
Each method is disputed as to which is actually quickest. My preference is the Act 5 chapter 5 but for variety I often mix it up.
New video, ignore the little ad at the start of it; it gives a good explanation on how to do it. Video credit: - austex360