Two for One
From Halopedia, the Halo wiki
Two for One is a Halo 3 multiplayer achievement. It is awarded for killing two enemies with one Spartan Laser shot in a ranked free for all playlist (i.e. Lone Wolves). Due to the difficulty in earning this particular achievement, people have been working together to let others earn this achievement. The achievement awards the player 5 Gamerpoints.
It is represented by a white circle with a red star. On top, there are two skulls and a Spartan Laser.
Walkthrough
Method 1
One way to get this achievement is to grab the Spartan Laser on Snowbound, head towards the back of the cavern which is the main "Arena" and make sure only the muzzle is sticking out of the door shield. Since more than likely all the players will be here and usually only take the tunnel entrances, they'll be too busy fighting each other to notice you charging up your laser.
Method 2
Another way to get this achievement is to also use the Spartan Laser on King of the Hill gametypes, where everyone will be grouped inside the hill and not able to pay attention to a Laser charging up in the background.
Method 3
One more way is plain and simple and is to have the Spartan Laser on maps that contain Ghosts, and all you would need to do is charge the laser and get the right moment for when someone boards the Ghost, using the Laser to destroy the Ghost will result in the driver and the boarder being "Lasered".
Method 4
And of course another way is, to ask somebody to help you with the achievement during a match.
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