This really isn't that hard, but it does take patience. In any Free For All game on a map that contains a Spartan Laser, grab the laser and try to stay out of harm's way. Hang out on the edges of the map, but where you can see the spots where action usually happens. Wait for two or more people to get into a short or mid-range combat situation and just blast away. Eventually you will get lucky and hit them both. I found this easiest on Snowbound or High Ground as they are both fairly small and contain Splazers.
Also this should be easier starting late September '08 as the playlists are changing and Free For All Slayer goes up from 6 players to 8 players!