Well First you need to play in Team Slayer then you need to play on one of these maps. -High Ground. -HC Snowbound. or if you have Mythic Map pack. -Avalanche.
What to do on High Ground. When the game starts you need to go direct to the Spartan laser and search for a good view point. I would advice the top of the base where the flag is positioned on one flag and just wait and watch. Otherwise the rocky cliff on the outside of the sniper cave.
What to do on HC Snowbound. When the game starts you will have take the Spartan laser in the base then position yourself behind one of the two power stations stationed just outside the base(Those big purple round things) . And just wait for the action to unfold in front of you. You can jump crouch on top of the grids for a even better view.
Some other notes. Sorry mark for deleting your info but it was just wrong. This is a ranked FFA achievement not a TS achievement. Som things to keep in mind when you are trying get this achievement is.
1. Posiotion yourself out of harms way preferably a high spot
2. Dont stay in sight al the time cause somebody will spot you and will try to aquire your shiny laser.
3. pick you battles if you want to get this achievement you will just have to let a few go cause you really need to line up the shot and thats kinda hard cause in lonewolves nobody tends to splash & bash with their AR. OK OK in the lower skill levels maybe :P.
And last but not least don't try to go john rambo with the laser it takes a whil to load so I suggest to keep on preloading the hell out of your laser. Cause you never know when those two dudes are going to pop up.
Best of luck getting the achievement