Over 1400 Halo 3 mongoose splatters
Try not to hit people in the back that are walking away from you. Laggy games usually make it harder. Try to stay as fast as possible and then hit someone head on or hit them with the side of your mongoose. It also helps to target people who don't notice you, that makes it the easist.
Its not too hard as long as you accept that your k/d might be bad when you try this, and you just keep trying until you get it. I have over 1200 mongoose splatters on this account so you can trust me.