Very hard. I only got mine recently.
Must be on lone wolves (all against all and ranked).
You can't do it with friends 'cos it has to be in ranked and no 2 people would let you do that to them just by asking. If they did say yes, then they are almost definitely lying. They will just kill you and the other guy as soon as you are near!
You can get the spartan laser on Snowbound (in one of the buildings) and Highground (on top of the left of the entrance to the base when you are approaching it from the bottom).
Most people say that Snowbound is easier, get the laser and keep charging it behind the laser gate of the big entrance to the central underground part. When you see someone inside the cave pop in and zap him! Hopefully there will be someone behind them or they will be fighting someone.
I got mine on Highground. I just kept trying to "snipe" people with it from the tower that you get it. Someone was trying to board an already occupied ghost, so I zapped the ghost and it killed them both! I was so happy!
Good luck! It takes a lot of time!
